Just upgraded my azure AD connect to the latest version and added back a customized rule that set's the msExchMailboxGuid to NULL as described in the article below. This was required for migrationwiz to work.
But I wonder if I enable the synchronization will it set the msExchMailboxGuid to NULL (in office365)? I've checkmarked the "apply once" but I have no idea if it "remembers" what has been synced in the past, I assume it does because it uses an SQL database?