Perhaps if you are going to lock the price as long as the subscription remains valid, you might also want to institute another way of changing payment method. Now to change from one credit card account to another I have to let the old subscription expire and resubscribe with a different card. Most online vendors allow one to change the account billed without cancelling the order and reordering.
Get the money when the opportunity arises. If you want to sit on it and let the opportunity pass you by well then that is your decision. BB