Microsoft has written a knowledge base article on how to do this. Here is a quote from the beginning of the article
You may want to move all of the users from one Exchange Server computer to another, often during a hardware or software upgrade. If the server has a large number of users who have large mailboxes, the move may take an unusually long time if you use the Move Mailbox utility of the Exchange Server Administrator program in the usual manner.
You can "forklift" all of the user data at once, if you physically move the Priv.edb file from one server to another. This significantly reduces the time that it takes to accomplish the entire move. This procedure is useful only if you move the users to a new server that does not already have user data on it, because it destroys all of the mailbox data that is already on the destination server.
Do not use this method to move public folder data. This method requires server down time. Allow at least two hours, plus any time that you need to back up, restore, or otherwise physically move your mailbox data.
The full article is
Q199954 - Forklifting All Users to a New Server.