The Complete Guide to Migrating From Microsoft 365

Migrating from Microsoft 365 is a significant undertaking, but with proper planning, it can be executed with minimal disruption to your workforce. This guide covers everything your IT team needs to know.

Phase 1: Assessment (Weeks 1-2)

Before any migration begins, you need a complete picture of your current Microsoft 365 usage.

What to Audit

• Mailbox sizes: Average and maximum mailbox sizes across your organisation
• Shared mailboxes and distribution groups: Document all shared resources
• Calendar complexity: Recurring meetings, room bookings, shared calendars
• SharePoint sites: Number, size, and usage patterns
• Teams channels: Active channels, integrations, and custom apps
• Third-party integrations: Apps connected to Microsoft Graph API
• Custom workflows: Power Automate flows, Power Apps

Key Questions

1. How many active users do you have?
2. What’s your total data volume (email + files)?
3. Do you have regulatory requirements for email retention?
4. Are there any legacy applications that depend on Exchange or SharePoint APIs?

Phase 2: Planning (Weeks 3-4)

Migration Strategy

We recommend a parallel run approach:

1. Set up UnionStack alongside your existing Microsoft 365 environment
2. Configure mail routing to deliver to both systems
3. Migrate historical data in the background
4. Switch primary mail routing to UnionStack
5. Decommission Microsoft 365 after a comfort period

User Communication

• Announce the migration at least 4 weeks in advance
• Provide training materials and documentation
• Designate “champions” in each department to provide peer support
• Set up a dedicated support channel for migration questions

Phase 3: Technical Migration (Weeks 5-8)

Email Migration

Our automated migration tools handle:

• Full mailbox content (emails, folders, rules)
• Calendar events (including recurring events and attendees)
• Contacts
• Shared mailboxes and distribution groups
• Email signatures and auto-replies

Typical speed: 50-100 GB per hour, depending on mailbox complexity.

File Migration

• SharePoint document libraries → UnionStack Drive
• OneDrive personal files → UnionStack personal storage
• Permissions are preserved during migration
• Version history is maintained

Identity

• Active Directory / Entra ID synchronisation is supported
• SAML/OIDC SSO configuration
• MFA policies carry over

Phase 4: Validation (Week 9)

• Verify all mailboxes are complete and functional
• Test calendar synchronisation
• Confirm file access and permissions
• Validate SSO and MFA
• Run user acceptance testing with department champions

Phase 5: Cutover (Week 10)

• Update MX records to point to UnionStack
• Redirect SharePoint URLs to UnionStack Drive
• Monitor support tickets closely for the first 48 hours
• Begin Microsoft 365 licence wind-down

Common Challenges and Solutions

Large mailboxes (>50 GB)

These take longer to migrate. Start with the largest mailboxes first and run migrations overnight.

Custom Teams integrations

Audit all custom integrations early. Most can be replicated with UnionStack’s API, but some may require re-development.

User resistance

Change management is often the biggest challenge. Focus on what users gain (data privacy, often faster performance) rather than what changes.

Timeline Summary

Phase	Duration	Key Activities
Assessment	2 weeks	Audit current environment
Planning	2 weeks	Strategy, communication
Migration	4 weeks	Data transfer, configuration
Validation	1 week	Testing, UAT
Cutover	1 week	DNS changes, go-live

Total: 10 weeks for a typical enterprise with 1,000-5,000 users.

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Ready to plan your migration? Contact our migration team for a free assessment.