M&A activity, divestitures, brand consolidations and tenant rationalisation all drive Microsoft 365 tenant-to-tenant migrations. They look simple on a slide. They're not. This is the field guide for UK IT teams running tenant migrations — what tools work, what timeline is realistic, and what breaks.
What's in scope
Mailboxes (Exchange Online) — most-visible workload.
OneDrive — per-user file shares.
SharePoint — team sites + document libraries.
Teams — chat history, channels, files, calls.
Identity (Entra ID) — user objects, groups, conditional access, MFA enrolment.
Intune — device enrolment, policies, app deployments.
Power Platform — Power Apps, Power Automate flows, Dataverse.
Any third-party apps using Entra ID for SSO.
The 4 migration paths
Tool-based migration — ShareGate, Quest On Demand Migration, BitTitan MigrationWiz, AvePoint Fly. Mature tools, well-tested. Typical choice for mid-market — see how this fits our EA vs CSP licensing programme decisions.
Microsoft-led FastTrack — Microsoft's own migration service. Free at qualifying spend levels. Best for E5-heavy customers (pairs naturally with Defender for Endpoint rollout and Entra ID hardening).
Native PowerShell scripting — custom scripts for specific workloads. Cheapest but operationally demanding.
Coexistence-first — accept long-running multi-tenant + use cross-tenant access settings. Increasingly viable since 2024 — useful when paired with ZTNA migration to remove flat-VPN dependencies.
Realistic timeline
Discovery + planning: 2-4 weeks. Tenant audit, user mapping, app inventory, identity sync planning.
Identity sync setup: 1-2 weeks. Cross-tenant access settings, B2B invitations or federated identity setup.
Mailbox migration: 2-6 weeks for 100-1,000 users (most tools migrate 50-200 mailboxes/day depending on size).
OneDrive + SharePoint: 4-8 weeks for typical estate. Larger if heavy permission inheritance complexity.
Teams: 2-4 weeks. Teams migration is the most-complex workload — chat history fidelity is imperfect across most tools.
Coexistence period: 30-90 days minimum. Both tenants live + cross-tenant access enabled.
Source tenant decommission: 30-60 days after last migration.
Total realistic: 3-6 months end-to-end for typical mid-market (500-2,000 user) migrations.
Common gotchas
Teams chat history — no tool migrates Teams chat with 100% fidelity. Set user expectations early.
Shared mailboxes + room mailboxes — easy to forget; check before cutover.
OneDrive sync — users on old tenant's OneDrive client need re-enrolment on new tenant.
Conditional Access — new tenant's Conditional Access policies need to match (or improve on) source tenant. Audit before cutover.
Third-party app SSO — every SaaS app federated to source tenant needs re-federation to target tenant. List + plan early.
What Servnet does
Servnet runs M365 tenant-to-tenant migrations using ShareGate / Quest / BitTitan tools, supported by Microsoft 365 + Entra ID-certified engineers.
Typical UK engagement: 3-6 months end-to-end, fixed-fee, includes tool licensing + Servnet engineer time + named project manager + 30-day post-cutover hypercare.