BES doesn't have an EXPORT/IMPORT feature for user accounts so there's no way to even do what you're suggesting.
Just keep the same SQL database and point the new BES to it. You won't need to reactivate the handhelds if they're using the same SQL database and SRP.
Basically, you want to do a knife-edge cutover.
http://www.blackberry.com/btsc/KB17029