you mean on BES?

i dont follow the wording of your question... i think you mean changing the user/PIN association.

yes no?

On BWC: you have to get the PIN cleared for the former owner - hard to do unless you know the acc name and phone number assigned before.
On BES I have no clue, probably it's much easier.

Can anyone advise me on how one can change the user which was assigned to a specific pin number so that the handheld can be reused with that pin number for a new user?

yes. I want to change the user and pin from a BIS service to be reprovisioned for another user.