Two methods exist to get message out of one Exchange data store and into another.
1. Run up the outlook client on each user’s PC and use it to export all messages from that user mailbox to an individual .pst file. Copy the .pst files across to the new server and then use the same functionality of the Outlook client to re-import them into the new system.
2. Microsoft have produced a utility called Exmerge. This utility allow syou at a server level to export specified or all users mailboxes to individual .pst files and then optionally delete the mailbox. Exmerge can then be used to re-import the mailbox into the new datastore on the new server and create the mailbox on the fly.