I stumbled across this about a month ago when my subscription ran out.
Just wait until your subscription expires then re-subscribe using your credit card.
Once it says successful leave the account management screen open and quickly log into WoW, once logged in go back to the account management screen and cancel your subscription.
The trick is once you re-subscribe it puts "Pending" next to your payment method while they do checks against your credit card etc, I?m not sure how long it stays like that but I?ve always cancelled my payment straight after I log into the game.
I've received no bills on my card and it works every time.
The reason i'm letting this out now is because I finally got banned(from trying and omg I went out with a bang) so I no longer care if others use this.