Microsoft 365 tenant-to-tenant migration checklist

A phased plan for merging or splitting Microsoft 365 tenants — identity, mail, OneDrive, Teams, licensing and the cutover runbook.

Tenant-to-tenant migrations happen after acquisitions, divestitures and rebrands. They are not technically hard; they fail on sequencing, domain ownership and communication.

Phase 1 — discovery

• Inventory users, shared and resource mailboxes, distribution groups, Microsoft 365 Groups, Teams, SharePoint sites and OneDrive volumes.
• List every custom domain and where its DNS is hosted, including any used only for mail relaying.
• Record licence SKUs on both sides — the target must have licences ready before mailboxes land.
• Find applications using the source tenant for SSO (Entra enterprise apps) and any conditional access dependencies.

Phase 2 — prepare the target

1. Create users (or sync them from the merged AD/Entra Connect) with the target's routing domain.
2. Assign licences and pre-provision OneDrive.
3. Build conditional access, DLP and retention policies to match the source, and test them with pilot accounts.
4. Set up mail flow coexistence — a shared address space and connectors so both tenants can route to each other during the overlap.

Phase 3 — pilot

Move 5–10 representative users covering each persona: heavy Teams user, shared-mailbox delegate, mobile-only user, external collaborator. Verify Outlook profile recreation, Teams chat availability, OneDrive sync and MFA re-registration. Every issue you find here would otherwise have been a hundred tickets at cutover.

Phase 4 — cutover

1. Lower DNS TTLs 48 hours ahead.
2. Freeze changes in the source tenant.
3. Run the final delta sync of mail and files.
4. Remove the domain from the source tenant (this is the long pole — remove all references first: users, groups, aliases, Teams).
5. Add and verify the domain in the target; update MX, SPF, DKIM, DMARC and autodiscover.
6. Re-point SSO applications and re-issue any app registrations.

What usually goes wrong

• Domain removal blocked by a forgotten alias or a soft-deleted object. Check soft-deleted users and groups.
• Teams chat history does not migrate natively — set expectations early or use a third-party tool.
• Guest access breaks for external collaborators; re-invite them after cutover.
• Free/busy gaps during coexistence if organisation relationships are not configured.
• Mobile devices need Intune re-enrolment; schedule a support desk surge.

Communication plan

Three notices — one week, one day, and morning-of — each stating the new sign-in address, what to expect on their phone, and the exact support channel. Migrations are judged by how well people were warned, not by how clean the delta sync was.