We migrated a company to our tenant a little over a week ago. They have about 50 users that have some sort of M365 account. We wiped 20 computers/re-enrolled them our tenant, and bought about 16 new computers enrolled in Autopilot.
This was a beat-down. Three of us + a relative of someone + two 'smart hands' from a local MSP ate through this in two days.
I can't imagine how a migrating hundreds of people would go. How are huge migrations done? The domain must be removed from one tenant and added to the other, so I am unclear how staging is done? Is it something where you force the user to have fabrikam.com instead of contoso.com and forward all their mail to Fabrikam from contoso until the tenant is moved? That is the only way I can see it done.
Given the size, we prepped everything, then removed the domain, swapped dns, added the domain to our tenant, and waited.
Source: r/sysadmin · by /u/bjc1960