Ummm, what happens if you cancel your credit card? On my PS3 i am changing between Entropay card and a prepaid card all the time. Man MS's policy is stupid. Its all for them and **** all for the consumer.
It just sits there. I know, because I have two entries put in. I guess it stops people charging things, since it defaults to the first number, and you have to go through a few options to change it.