Yay, just done this.
I phoned MS today to try and get access to my old GT which is associated with an email address I can't access (don't know password/question), and unfortunately need to wait till next working week to get further help. Security question form to try and regain access failed.
So I just purchased on my current live account (which now has a fresh default GT) and if I get access back to my old GT I'll switch out it's name and reassign it to current.
Playing just now with a random default Xbox GT (OutlawQuasar481 lol). My Audioboxer GT only has a few thousand gamer points (no big loss).
Is there anything that can be picked up off the store with the change from 8000 yen?