When an admin disables a member account, the User Type should be returned to the available pool. This is an absolute no-brainer in my opinion.
When admin tries to re-activate an account, if there are no available User Types in the pool, simply give them an error pop-up and the account stays disabled. If there are available User Types, prompt them to select one (also give option to cancel and remain disabled if they don't see one they want to assign.) If they select one, re-activate member account and apply User Type.
Your welcome.