You can only directly copy the mailbox directories if the new server has the same processor than the old one. Otherwise you're going into trouble. If you change teh processor the the easiest way is to use a tool like imapsync. This tool allows you to sync to imap mailboxes, so you can sync the old mailbox with the new mailbox on the new DS. imapsync runs on Linux and Windows. You can even compile it to run on a diskstation