in my experience there are 2 answers:
1. if u upgrade from 10-day trial account u have the option to pay using game card(like me)
2. if u upgrade from trial that was sent by a friend (recruit a friend) u need to have a credit card to be able to upgrade to full account ( i can confirm this coz i helped a friend into wow and when upgrading he had to deal with the whole credit card thing )

hope this helps somewhat

Edit: btw no.2 was in november 2007 so im not sure if its still the same

This post has been edited by EdG3: Jul 22 2008, 05:49 PM

did he buy a new game or upgraded from recruit-a-friend?

yea both needs a key to upgrade to full account but the recruit-a-friend trial will not allow u to pay game time using game card, u will need a cc.

This post has been edited by EdG3: Jul 22 2008, 06:19 PM

u get 30days to play after upgrading to full account. after that only need to use gamecard or pay by cc