Moving Mailboxes Using Exchange System Manager

You can move a mailbox to another store if necessary. Using Exchange Task Wizard select as many mailboxes as you want to move and then, using the task scheduler, schedule the move to occur at some time in the future. You can also use the scheduler to cancel any unfinished moves at a selected time. For example, you can schedule to move a large number of mailboxes at midnight on Friday and automatically end at 6:00 A.M. on Monday, thereby preventing your server's resources from being depleted during regular business hours. Using the wizard's multithreaded capabilities, you can move up to four mailboxes at the same time.

For detailed instructions, see "How to Move Mailboxes from One Exchange Virtual Server to Another Server"