When you add a new card to your account and set it as the default, it is used as the default card for new orders only.
Your old card will still be the default card for any orders placed with it, unless you make a "Pay Now" payment and select the new card as the default.
If a payment for an order fails, we will try other cards on your account. Once all associated orders with a card are paid off, you can remove the old card from your account in the My Account / Billing section.
When adding a new card please ensure that the card has been activated with the bank and that you have at least $5 available to spend.