Hello there!!
Thanks for sharing that. Do you know Connecting Software's CB Exchange Server Sync?
It's the solution I work with and what I've heard from our customers is that they benefit from a soft migration, meaning that first they sync the data from mailbox A to mailbox B, and they only delete mailbox A (and hence finish the migration) when they are 100% confident with it.
You can check it out at
https://www.connecting-software.com/cb-exchange-server-sync/!