You can do it, as I had to so I could play CS again...
What you have to do is get to your profile details on the 360, and make sure you write all the information down exactly (credit card #, name, address...everything).
Then do an account recovery on the Xbox. Plug in all the details from the 360's account exactly into the Xbox, and it will be fine.
If not, there's always 1-800-4MY-XBOX |