A while back I got a game card from a family member for the game, and it would not let me change my payment info until my current sub ran out, in which I had to re-activate it under the option of having 8 characters/world with up to 40 characters on said account, when originally I only had it as character/world, so as to what I'm aware of, I don't think you can change payment type until the sub runs out.

I may be wrong, but this is what I've seen.