How to change / edit existing subscriptions payment method?

Hi,

I am understanding that since you "want to use a different card" but do not want to "add a payment method" you simply wish to change the order in which cards are charged. Go to the place where you see your payment methods listed. The Add a payment method to your Apple ID - Apple Support document will show you how to do that. You can then move a payment method lower on the list to a higher position.

I don't use Apple Pay so I don't know how it appears in the payment list. If it is there and it is a valid payment method for your country, then move it up the ranking.