I'm just a community volunteer and I might be entirely wrong, but from what I remember there's a system called Sneakernet which is supposed to be able to handle gamertag changes from after the halo stat freeze so that you can still be connected to your halo stats and such. From my understanding though that system broke at some point last year and they haven't gotten around to fixing it. Perhaps later I can dig around and try to find the post that verifies this.