No mate, once your gamertag is assigned to an address, it is stuck. They reckon they are bringing out a way to change this with the update later in the year
There would be a way round it, although it's stupid :
Pay 800 points and change your gamertag to something else, then register a new gamertag, the same as your current one, and assign that to whatever email address you like. You lose all your achievment points obviously on the new tag, and it costs 800 points, which makes it gash
Depends how much you want your current gamertag\score I guess, and i'm guessing you've got gold subscription on it too, which I think you can get a part refund for