Account Settings => Payment Plans => Edit => <select new card> => OK
Does not change card.
Account Settings => Payment Plans => Edit => <select new card> => Edit
Goes back to Payment Plans screen where the History & Edit buttons are clickable but inop. (Chrome)
Firefox is a bit flakier on that last operation. One time History worked, but Edit didn't. Another time History & Edit worked, but the Edit screen buttons were inop. Most times it works, except for allowing the card used for payment plan to change.