Let customers change the payment method an active subscription uses

Problem/Motivation

If I need to change the credit card I'm using for a subscription, while I can add it to my user account and make it the default for my user, it does not update any existing subscription. They still refer to the old payment method, and even if I delete the old payment method, any subscriptions using it will not be updated to use my new default payment method.

Proposed resolution

As a baseline, when a customer changes the default payment method on their account, I think any active subscription should be switched to using this new payment method. We could support retaining the old payment method and make it optional, but whether it's automated or optional, we should ensure the "Make default" form explains the implications.