I to am probably the same as Kevin Whelan but we use O365.
Didn't use their old service and I just made sure everything was matching before hitting the switch.
Backup first, disable DirSync and then view the outcome in AD. Students simply moved into the Kamar OU structure, no loss of groups attachment, just a quick reconfigure of group policy.
I then run a batch file on new student which calls a powershell script to populate pager, employeeID, exchAttribute fields for which I use for other services like papercut, accessit etc.
Password sync disabled. I pretty much don't do anything with new enrollments now.
Matt