We are currently testing migrating users from one mailbox system to another. The zmmailboxmove util. seems to do the trick, with one exception.
We are finding that once we move accounts to the new mailbox server, all of the user's IM Buddies are gone. You can go about re-adding each one, but until that is done the user doesn't appear online, etc. to other users on the system, both on the original and new mailbox server.