It isn't necessary to do that - provided you keep the old Exchange Server running after you have migrated everything off, then when users open Outlook for the first time after the change, it will connect to the old server, realise the mailbox no longer exists there and locate the mailbox on the new server. The account configuration in Outlook will be automatically updated for the new server name, meaning after a week or so, you can safely remove the Exchange role from that server.
-tigermatt