How Do You Prepare a Large Microsoft 365 Tenant-to-Tenant Migration?

I’m researching the best way to prepare for a large Microsoft 365 tenant migration and would appreciate some practical advice from people who have handled one before. The migration itself seems like only one part of the project. Preparing both tenants properly appears to be just as important.

One recommendation I keep seeing is to start with a complete inventory of the source environment. This includes identifying active mailboxes, mailbox sizes, user accounts, aliases, permissions, retention policies, and other configurations that could affect the migration. The destination tenant should also be prepared before transferring any production data.

I’m especially interested in Cross-Tenant Mailbox Migration because moving a small number of accounts seems relatively straightforward, while larger migrations require much more coordination. A pilot migration could help identify mapping or authentication issues before thousands of users are involved.

Another thing I’m considering is how administrators should handle the actual transfer. Microsoft provides native migration capabilities, but the process can involve PowerShell configuration, application permissions, endpoints, organizational relationships, and CSV-based mailbox mapping. That seems manageable for experienced administrators but could become complicated for a large project.

I also found the SysInfo Microsoft 365 Tenant to Tenant Migration Tool as an alternative approach. From what I understand, it can connect source and destination tenants using modern authentication, map accounts, apply migration filters, and provide task monitoring and reports.

For organizations planning Cross Tenant Migration Office 365, what preparation steps made the biggest difference for you? Did you perform a pilot first, create a detailed rollback plan, or migrate users in smaller batches? I’d be interested in hearing what worked and what you would do differently next time.