You'd have to read the small print on the cards, I'vea feeling most of them are for new users only.
However, theres nothing to stop you making a new account using it and playing with that.
Rock Band next week! I challenge you to a DRUM OFF!

Also there are a few cheap ones flying around the usual auction site where you get 13 months for £24-£26
I used one the other month with no problems and you dont have to have the code shipped by email you can get them to send you the scratch card.
