Going through Microsoft's TechNet and other tutorials they all seem to vary on how to do this. Let's say there are 70 users and you only want to do 5 users at a crack.
Exchange 2003 needs to stay running and the 5 users need to be able to send/receive email from Exchange Online. They mentioned about changing the TargetAddress to their Office 365 email, but if it is the same email address then how on earth would it know where to go if the MX record is still pointing at the Ex2003 box?
Could you technically add a secondary MX record with a higher priority that goes to their Office 365 account and just delete the mailbox (not the AD user) so that when any new incoming email comes into the first MX it won't see a mailbox, but then go to the second one?