Can’t change method of payment
How do you change method of payment if a card that was declined on an active subscription and you want to replace it?
If there is a pending payment (on the old card), I suggest that you contact iTunes Store Support Team directly, using this specific link:--> Apple Support (if you get no help, look at the bottom of the above link, where you can contact them using "chat" or "phone" options).
Regards
Giulio