Prepaid game card

I had just purchased a prepaid game card in order to play WoW for two months without using a credit card, but after buying it and bringing it home, i discovered that you must actually buy a retail version of the game to upgrade your account from a trial version (which i have). Now, i was wondering if there was any possible way to upgrade my account with the prepaid game card, If not, i do have an old retail version of the game, but i had already used the code before to upgrade an old account which I had cancelled (not enough spare time to play) and i think that its ridiculous if I actually did have to buy yet another, retail version of the game while i already have the game discs and my old code.

My question is: Given these circumstances, is there ANY possible alternitive for me to upgrade my trial account, short of buying another copy of the retail game?

-any help would be greatly appreciated