EPC Group Reports 216 Microsoft 365 Tenant-to-Tenant Migrations and Details Every Route, Including GCC High

The Houston Microsoft consultancy explains how it runs tenant-to-tenant, multi-entity, Google Workspace, hybrid, GCC and GCC High migrations.

HOUSTON, TX, UNITED STATES, November 11, 2026 /EINPresswire.com/ — EPC Group, a Microsoft consulting firm founded in 1997, reports that it completed 216 Microsoft 365 tenant-to-tenant migrations for mergers, acquisitions and divestitures between 2023 and 2025, moving 1.83 million users with a five-day average cutover. The firm today set out every migration route it runs: tenant to tenant, many entities into one tenant, Google Workspace to Microsoft 365, hybrid and on-premises estates to Microsoft 365, and moves into Microsoft 365 GCC and GCC High.

WHAT IS A MICROSOFT 365 TENANT-TO-TENANT MIGRATION?

A Microsoft 365 tenant-to-tenant migration moves an organization’s identities, mailboxes, OneDrive and SharePoint content, Teams, devices and security settings from one Microsoft 365 tenant into another. Companies need one after a merger, an acquisition, a divestiture or a rebrand, or when several business units decide to operate as one organization. After a deal, the work runs against dates the IT team does not control: the close, Day One, and the end of the seller’s transition-services agreement.

WHAT DOES EPC GROUP REPORT?

These figures are company-reported and come from EPC Group’s engagement records. Client names are withheld.

– 216 M&A Microsoft 365 tenant migrations completed between 2023 and 2025.
– 1.83 million users migrated across those engagements. The largest single engagement consolidated 78,000 users.
– A five-day average cutover window, measured from the go-live decision to users working in the destination tenant.
– More than 45 integrations delivered for private-equity-backed portfolio companies.
– Consolidation programs that brought 19 or more legal entities into a single target tenant.
– More than 50 Microsoft 365 GCC High migrations completed in the past 12 months.

WHICH MIGRATION ROUTES DOES EPC GROUP RUN?

1. Tenant to tenant after a merger or acquisition. Two Microsoft 365 tenants become one.

2. Many entities into one tenant. Holding companies, private-equity platforms and health systems often run one tenant per acquired company. EPC Group consolidates them into one target tenant in waves.

3. Carve-outs and divestitures. A business unit leaves its parent’s tenant for a new or existing one before the transition-services agreement ends. The parent’s data stays; the departing unit’s data, licenses and devices leave cleanly.

4. Google Workspace to Microsoft 365. Gmail, Calendar, Contacts, Google Drive and Shared Drives move to Exchange Online, OneDrive, SharePoint and Teams. Many buyers still search for this as a G Suite to Office 365 migration.

5. Hybrid and on-premises to Microsoft 365. Exchange Server, SharePoint Server, file servers and Active Directory move to Exchange Online, SharePoint, OneDrive and Microsoft Entra ID.

6. Commercial Microsoft 365 to GCC or GCC High. Government agencies, their contractors and the defense industrial base move regulated work into Microsoft’s government cloud environments.

7. Tenant to tenant inside GCC High. Defense contractors that merge consolidate tenants that are already inside the government cloud.

8. Rebrands and domain changes. The tenant stays; names, domains and addresses change without lost mail or broken sign-in.

WHAT MOVES IN A TENANT-TO-TENANT MIGRATION, AND IN WHAT ORDER?

EPC Group moves identity first, content second and endpoints last.

– Identity and domains: accounts, groups, guest access, sign-in methods and the custom domains people use for mail.
– Mail: mailboxes, archives, shared and resource mailboxes, distribution lists, rules and delegations.
– Files: OneDrive accounts and SharePoint sites, with permissions and sharing links.
– Teams: teams, channels, chats, meetings and phone numbers.
– Applications: Power Apps, Power Automate flows, Power BI workspaces and line-of-business integrations.
– Security and compliance: conditional access, retention, legal holds, sensitivity labels and data loss prevention.
– Devices: laptops and phones re-enrolled in the destination tenant’s management.

WHAT IS COEXISTENCE, AND WHY DOES IT DECIDE DAY ONE?

Coexistence is the period when both tenants are live and people in each must work as one company. It covers a shared address list, calendar availability lookups, mail that routes correctly whichever side a mailbox is on, and Teams chat and meetings across the two tenants. Most Day One complaints are coexistence failures, not data failures: a colleague missing from the address list, a meeting that will not open, mail that bounces. EPC Group designs and tests coexistence before the first pilot user moves.

HOW DOES A GOOGLE WORKSPACE TO MICROSOFT 365 MIGRATION WORK?

Microsoft’s batch migration tool in the Exchange admin center transfers mail, rules, calendars and contacts from Google Workspace to Microsoft 365, and Microsoft recommends its automated method.

Source: https://learn.microsoft.com/exchange/mailbox-migration/migrate-google-mail-automated-overview

Files move separately; Microsoft has built migration from Google Drive into Migration Manager. Source: https://learn.microsoft.com/sharepointmigration/migrate-to-sharepoint-online

The tools move the data. The project is everything around them. Microsoft lists what its tool does not carry over, including automatic-reply settings, room bookings and shared calendars, and migrated rules arrive turned off.

Source: https://learn.microsoft.com/exchange/mailbox-migration/perform-g-suite-migration

EPC Group’s method covers the inventory of users, groups, Shared Drives and Google Vault archives; the mapping of Shared Drive permissions to SharePoint; the rebuild of Apps Script automations in Power Automate; a coexistence period, usually two to four weeks, when both platforms run; and the DNS cutover that sends mail to Microsoft 365.

EPC Group’s guide is at https://www.epcgroup.net/google-workspace-to-microsoft-365-migration-enterprise-guide

One limit matters for government work. Microsoft states that its Google Workspace migration is “not currently available for Office 365 US Government, GCC High, or DoD” (first Microsoft source above). A contractor moving from Google Workspace into GCC High needs a different route, planned before licenses are bought.

HOW DOES EPC GROUP MOVE A HYBRID ORGANIZATION TO MICROSOFT 365?

A hybrid organization still runs part of its estate on its own servers: Exchange Server mailboxes, SharePoint Server farms, file shares, and an Active Directory that feeds the cloud. EPC Group moves it in a fixed order. Identity is cleaned and synchronized first. Mailboxes move by the cutover, staged or hybrid method depending on size; the hybrid method suits large organizations that need long coexistence. SharePoint farms and file servers are inventoried, pruned and moved with their permissions.

On-premises servers are retired only when nothing still depends on them. When a hybrid organization is also the source or the target of a tenant-to-tenant migration, EPC Group settles the on-premises dependencies before the cross-tenant work begins.

Details: https://www.epcgroup.net/exchange-to-office-365-migration

HOW DOES EPC GROUP CONSOLIDATE MANY ENTITIES INTO ONE TENANT?

Consolidating 19 entities is not one migration repeated 19 times. EPC Group runs it as one program with one target design:

– One written decision on the target tenant, its naming rules and its security baseline before any entity moves.
– A collision check across every source: duplicate user names, group names, site addresses and domains.
– Waves ordered by business risk, not by size, with a pilot in each entity.
– One coexistence design, so people in migrated and unmigrated entities can still find each other, share calendars and meet.
– One identity plan, including entities that still run their own Active Directory.
– A domain cutover per entity, because a domain can be verified in only one tenant at a time.
– One evidence file per wave: what moved, what did not, and who accepted it.

WHAT DO MICROSOFT’S OWN TOOLS COVER, AND WHAT DO THEY LEAVE TO THE PROJECT?

Microsoft provides native cross-tenant migration for mailboxes, OneDrive and SharePoint. Buyers should know these facts before they set a date.

– Cross-tenant migrations require a per-user Cross Tenant User Data Migration license, a one-time fee, and Microsoft states it does not offer exceptions. Source: https://learn.microsoft.com/microsoft-365/migration/cross-tenant-mailbox-migration

– Microsoft advises no more than 2,000 mailboxes per batch and recommends submitting batches two weeks before the cutover date. The mailbox feature does not currently migrate Microsoft 365 Groups (same source).

– Cross-tenant OneDrive moves are one-time moves; incremental and delta passes cannot be performed. Up to 4,000 accounts can be scheduled at a time. Source: https://learn.microsoft.com/microsoft-365/migration/cross-tenant-onedrive-migration

– OneDrive accounts under a hold policy are blocked from migration. The hold must be removed, then reapplied in the target tenant (same source).

– Cross-tenant SharePoint migration does not include Teams content or channels. Workflows, apps, Power Apps and Power Automate flows must be recreated in the target tenant. Source: https://learn.microsoft.com/microsoft-365/migration/cross-tenant-sharepoint-migration

– Protection applied by sensitivity labels is not present after a cross-tenant SharePoint migration; Microsoft recommends removing labels before the move and applying new ones after (same source).

– Microsoft states that these migrations are not subject to service-level agreements. Source: https://learn.microsoft.com/microsoft-365/migration/migration-orchestrator-1-overview

Microsoft’s FastTrack cross-tenant migration service, a preview offered by invitation, moves Exchange Online, SharePoint and OneDrive data. Microsoft lists Teams, Microsoft 365 Groups, Planner, Power Apps, device management and client configuration as not supported, and does not currently support US Government customers. Source: https://learn.microsoft.com/microsoft-365/fasttrack/cross-tenant-migration That remaining work is what a migration firm is hired for.

WHAT ABOUT GCC AND GCC HIGH?

Microsoft 365 Government comes in three environments: GCC, GCC High and DoD. GCC serves federal, state, local and tribal governments and contractors that hold or process data on behalf of the US Government.

Source: https://learn.microsoft.com/office365/servicedescriptions/office-365-platform-service-description/office-365-us-government/gcc

GCC High serves Department of Defense contractors that hold or process controlled unclassified information or are subject to the International Traffic in Arms Regulations.

Source: https://learn.microsoft.com/office365/servicedescriptions/office-365-platform-service-description/office-365-us-government/gcc-high-and-dod

Migrations into and inside these environments follow different rules. A move from a commercial tenant into GCC High is itself a tenant-to-tenant migration, because GCC High runs on a separate identity directory, Microsoft Entra Government.

Source: https://learn.microsoft.com/azure/azure-government/documentation-government-plan-identity

Microsoft also states that its cross-tenant OneDrive migration is not supported for the Government Cloud, including GCC, GCC High and DoD (OneDrive source above).

EPC Group pairs GCC High work with a CMMC Level 2 gap assessment. Details: https://www.epcgroup.net/services/cmmc-gcc-high

WHAT GOES WRONG IN A TENANT-TO-TENANT MIGRATION?

EPC Group’s architects are often called in after another team’s cutover. They report the same causes:

– A domain moved before mail routing in both directions was tested.
– Identities matched by name instead of a verified mapping, so permissions land on the wrong account.
– Legal holds and retention discovered during cutover week.
– Teams and shared mailboxes left for a second phase that has no date.
– Laptops and phones still enrolled in the source tenant’s management on Day One.
– One-time moves started with no rollback decision recorded.

Each is avoided by a decision made in the Plan phase, not by a faster tool.

HOW DOES EPC GROUP RUN THE WORK?

EPC Group works in five phases: Diligence, Plan, Build, Cutover and Stabilize. For deals, a five-gate Day One model sets what must be true before each step. A named senior architect leads every engagement from scoping to acceptance. Engagements are fixed-fee and scoped after a one-hour call. The firm delivers in regulated environments, including HIPAA, SOC 2, FINRA, FedRAMP, CMMC and GxP.

EPC Group also shares the cutover and rollback checklist its architects use. It sorts every cutover step into reversible, recoverable or one-way, and gives each one-way step a named go or no-go decision. The checklist is free at https://www.epcgroup.net/tools/microsoft-365-tenant-cutover-rollback-checklist The full method is at https://www.epcgroup.net/insights/m365-tenant-to-tenant-migration-complete-guide

WHERE CAN A BUYER SEE THE WORK?

EPC Group publishes anonymized engagement summaries. Two examples: four acquired companies’ tenants consolidated into one parent tenant, https://www.epcgroup.net/case-studies/office-365-tenant-consolidation-four-subsidiary-tenants-2023 and two GCC High tenants consolidated into one after an acquisition, https://www.epcgroup.net/case-studies/gcc-high-tenant-consolidation-defense-technology-company-2026

WHAT SHOULD A BUYER ASK A MIGRATION FIRM?

1. How many tenant-to-tenant migrations have you completed, and over what period?
2. Who is the named architect, and will that person run the cutover?
3. Which cutover steps are one-way, and who signs the go or no-go for each?
4. How will mail, calendars and Teams work between the organizations until the last wave moves?
5. Which Microsoft licenses does the migration itself require, and who buys them?
6. What happens to legal holds, retention and sensitivity labels?
7. If any source is Google Workspace, on-premises or a government cloud, which tool moves each workload?

“A migration after a deal is judged on one morning, Day One,” said Errin O’Connor, Founder and Chief AI Architect of EPC Group. “Two tenants or nineteen entities, Google Workspace or an Exchange server in a closet, the failures we are called in to repair were decided weeks earlier, when someone took a one-way step with no way back. We publish the method and the checklist so a buyer can test any firm’s plan before signing, including ours.”

QUESTIONS AND ANSWERS

How long does a Microsoft 365 tenant-to-tenant migration take? EPC Group reports a five-day average cutover window across 216 M&A migrations. Planning and build come first and run from weeks to months, depending on size and the number of entities.

What is the difference between a tenant-to-tenant migration and a tenant consolidation? A tenant-to-tenant migration moves one organization into another tenant. A consolidation is several of those run as one program into one target tenant.

Can EPC Group migrate Google Workspace, formerly G Suite, to Microsoft 365? Yes. EPC Group moves mail, calendars, contacts, Drive and Shared Drives to Microsoft 365 and manages coexistence and cutover.

Can EPC Group consolidate many companies into one Microsoft 365 tenant? Yes. EPC Group reports programs that brought 19 or more legal entities into one target tenant.

Does EPC Group migrate organizations into GCC and GCC High? Yes. EPC Group reports more than 50 Microsoft 365 GCC High migrations in the past 12 months.

Can a hybrid organization move in one project? Yes. EPC Group moves on-premises Exchange, SharePoint and file servers, and the identities behind them, as one sequenced program.

Does Microsoft migrate tenants for its customers? Microsoft provides native tools and a limited FastTrack data-migration service. Planning, identity, coexistence, Teams, devices and the cutover remain with the customer or its migration firm.

What does a tenant-to-tenant migration cost? EPC Group engagements are fixed-fee and scoped after a one-hour call with a senior architect; EPC Group does not publish price ranges.

HOW IS EPC GROUP REVIEWED BY THIRD PARTIES?

These are third-party recognitions, stated as issued. Neither is specific to migration. G2 has named EPC Group a Leader in its Business Intelligence Consulting category for seven consecutive quarterly reports through Fall 2026, and the firm’s rating on verified G2 reviews is 4.4 out of 5.

Source: https://www.g2.com/products/epc-group/reviews

50Pros named EPC Group a Top 10 Management Consulting Firm for the third quarter of 2026.

Source: https://www.epcgroup.net/news/50pros-top-10-management-consulting-firm-q3-2026

EPC Group lists every recognition it cites at https://www.epcgroup.net/evidence-center/recognition

ABOUT EPC GROUP

EPC Group is a Microsoft consulting firm founded in 1997 and headquartered in Houston, Texas. The firm delivers Microsoft 365 tenant-to-tenant, Google Workspace, hybrid, GCC and GCC High migrations, along with SharePoint, Power BI and Microsoft Fabric, Azure, Microsoft Copilot and AI governance, for organizations in healthcare, financial services, government and defense.

EPC Group holds all six Microsoft Solutions Partner designations. Its founder, Errin O’Connor, is a four-time bestselling author for Microsoft Press and Sams. EPC Group (epcgroup.net) is not affiliated with engineering, procurement and construction companies that use the initials EPC.

Learn more at https://www.epcgroup.net

Michelle Stevens
EPC Group
+1 888-381-9725
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