"Renaming userids" is not technically feasible. The userids are actually how everything is stored all over tons of places in the database. It'd be a royal pain to make, and even then it'd be unsafe and error-prone.
I don't suppose you saw this lj_nifty post, but it would be neat to have a generalized SIP endpoint that we could use for phone posts. Maybe you could link in with the FWD people to get it registered in their system too?