Preparing for Microsoft 365 to Google Workspace Migration

Migrating an entire organization from Microsoft 365 to Google Workspace is a high-stakes project. For an IT Administrator, success isn't just about moving data; it's about maintaining permissions, ensuring zero downtime, and managing user expectations.

This guide outlines a comprehensive, 600+ word technical roadmap for migrating Mailboxes (Outlook to Gmail) and Cloud Storage (OneDrive to Google Drive).

Phase 1: Pre-Migration & Discovery**

Before touching a single byte of data, you must audit your source environment.

Inventory Audit: Export a list of all active users, shared mailboxes, and OneDrive storage totals from the M365 Admin Center. Note any OneDrive accounts exceeding 100GB, as these may require specialized handling or "slimming down."

Provision the Target: Set up your Google Workspace tenant. Ensure all user accounts are created and that their primary email addresses (or aliases) match the M365 source.

Cleanup: Instruct users to empty their "Deleted Items" and "Junk" folders. In OneDrive, ask them to delete old versions or temporary "temp" files to reduce the migration payload.

Phase 2: Technical Setup (Google Admin Console)

Google provides native tools for this transition. The most robust option for admins is the Data Migration Service (New).

Setting up the Connector
In the Google Admin Console, navigate to Data > Data import & export > Data migration (New).

Select Microsoft Exchange Online (for Mail) and Microsoft OneDrive (for files) as your sources.

You will need to sign in with a Global Admin account for M365 to grant the necessary OAuth permissions.

Permissions & Domain-Wide Delegation
To allow Google to "read" your Microsoft data without requiring every user’s password, you must authorize the Google Migration App within the Microsoft Azure/Entra ID portal. This creates a service-level bridge between the two clouds.

Phase 3: Mailbox Migration (Outlook to Gmail)

Unlike Outlook, Gmail uses Labels instead of folders. During migration, your Outlook folder hierarchy is converted into a nested label system (e.g., Inbox/ProjectA).

Step-by-Step Execution:

Identity Mapping: Create a CSV file with two columns: Source Email (M365) and Destination Email (Google).

Filter Settings: Decide if you want to migrate everything or just the last year of data. Most admins perform a Bulk Migration of historical data first, then a Delta Sync later.

The "Pre-Stage" Pass: Start the migration while users are still working in Outlook. This moves 95% of the data in the background.

Handling Attachments: Be aware that Gmail has a 25MB limit for attachments. Large files in Outlook may fail or be skipped; the migration report will highlight these.

Phase 4: Drive Migration (OneDrive to Google Drive)

OneDrive and Google Drive handle permissions differently. OneDrive is "file-centric," while Google Drive is "permission-centric."

Key Considerations:

Permissions: Google’s native tool attempts to map "Read/Write" permissions to "Viewer/Editor." External sharing permissions (files shared with people outside your domain) often do not migrate and will need to be re-shared post-migration.

File Format Conversion: Decide whether to keep files as .docx/.xlsx or convert them to Google Docs/Sheets. For most businesses, keeping the original Microsoft format is safer to avoid formatting breaks.

Execution:

Map OneDrive Users: Similar to the mailbox CSV, map source OneDrive accounts to target Google Drives.

Run Scans: Use the Google Migration tool to "Scan" the source OneDrive accounts first. This identifies "disallowed" characters in file names (like ~, #, %) that Google Drive might reject.

Start the Transfer: Execute the transfer in batches (e.g., 50 users at a time) to avoid hitting Microsoft’s API throttling limits.

Phase 5: The Cutover (Going Live)

Once the bulk of the data is in Google Workspace, it’s time to flip the switch.

MX Record Change: Update your DNS settings. Change your MX records to point to smtp.google.com. Once this propagates, new emails will land in Gmail.

The Final Delta Sync: Run one last migration pass to catch the "scraps"—the emails and files created between the start of the migration and the DNS change.

Disable M365 Services: Set M365 mailboxes to "Blocked" or "Read-Only" to prevent users from accidentally sending mail from the old system.

Phase 6: Post-Migration Support

Outlook Users: For users who refuse to leave the Outlook interface, install Google Workspace Sync for Microsoft Outlook (GWSMO). This allows Outlook to talk to Gmail’s servers.

Training: Provide a "Gmail vs Outlook" cheat sheet. Explain that "Archiving" in Gmail is not "Deleting."

Validation: Verify that Shared Mailboxes and Calendar delegates were moved correctly, as these often require manual re-linking.

Community