If you're talking about a different Exchange organisation I think you might be looking at a BES reinstall. You should export your user list from the current BES and import it in to the new BES instance (that'll save you some time), then run the handheldcleanup.exe to ensure the imported BES users can connect to the new or migrated mailboxes. Devices should reactivate themselves automatically (if they're all on 4.x with a 4.x BES server). I have not tried this personally but this is how I percieve it. Others here may have a tried and tested method and can elaborate.