You have two possible ways to change the bank card used for your subscription.
Cancel the current subscription, and when its validity period ends, take out a new subscription using your new card.
If you plan to purchase a volume which is not included in the subscription, you can save a new bank card when you buy it. When confirming payment, make sure that the following option is selected :
“I authorize Ingenico to store my financial details in a secure way, so that I will not have to enter them again for future orders with this merchant.” Then click on the gearwheel icon at the top right-hand corner and on “Payment method” on the left. You can then select the bank card you want to use to pay for your subscription in “Card registered in your account”.