M365 Migration Planner

Migration timelines vary significantly based on organisation size, data volume, and complexity. A small organisation (<50 users) can complete a cutover migration in 1-2 weeks. Medium organisations (500-5,000 users) typically need 8-16 weeks for a phased approach. Large enterprises (50,000+ users) may require 6-12 months. Key factors include data volume (Exchange migrates at ~10-15 GB/hour, SharePoint at ~5-10 GB/hour), number of workloads, custom applications, and compliance requirements. Always include a 2-4 week pilot phase before production migration.

Microsoft provides native tools for tenant-to-tenant migration: Cross-Tenant Mailbox Migration (MRS) for Exchange, Migration Manager for files, Cross-Tenant Teams Migration, and Entra ID Cross-Tenant Synchronisation for identity. For complex scenarios requiring chat history migration, full coexistence, or Power Platform migration, third-party tools like Quest On Demand Migration, BitTitan MigrationWiz, or ShareGate may be needed. The choice depends on your workload scope, coexistence requirements, and budget.

Native Microsoft migration tools are free with your M365 subscription. Third-party tools typically cost $8-15 per user. Professional services range from $5,000-$50,000+ depending on complexity. Key cost factors include: tool licensing, internal IT labour hours, external consultant fees, training and change management, dual licensing during coexistence, and post-migration support. A small organisation might spend $2,000-$10,000 total, while a large enterprise could invest $100,000-$500,000+.

The top migration risks are: (1) Data loss — mitigate with pilot testing and post-migration validation. (2) Authentication disruption — plan MFA re-registration and SSO testing. (3) Permission escalation — audit permissions pre and post-migration. (4) Legal hold breaks — work with legal team before migrating held content. (5) Custom app failures — inventory all integrations and test in pilot. (6) Service throttling — plan for reduced throughput and schedule off-peak. (7) User disruption — invest in change management and training.

Yes. Microsoft's Migration Manager supports Google Workspace to Microsoft 365 migration for email (Gmail to Exchange Online), files (Google Drive to OneDrive/SharePoint), and contacts/calendars. Google Groups can be migrated to M365 Groups. However, some Google-specific features don't have direct equivalents: Google Sites migration is limited, Google Forms must be recreated in Microsoft Forms, and Google Chat history requires third-party tools. Plan for significant user retraining as the interfaces are very different.

The best approach depends on your size and complexity: Cutover migration (all at once) works for <150 users with simple environments. Staged migration (batches over weeks) suits medium organisations. Phased migration (workload by workload) is best for large organisations or M&A scenarios. Hybrid (coexistence) suits organisations with regulatory constraints. Always start with a pilot of 5-10% of users regardless of approach. Consider deadline pressure, compliance requirements, and user tolerance for disruption when choosing.

Best practice is to maintain the source environment in read-only mode for 30-90 days after migration. This provides a safety net for missed data, allows verification, and supports legal discovery requirements. After the retention period, you should: confirm all data is migrated and validated, ensure no active legal holds reference source content, verify no applications still point to source, export audit logs, and then decommission. For tenant-to-tenant migrations, domain transfer must complete before the source tenant can be fully decommissioned.