@Matthew671 I had the exact same situation last year where the credit card used for the first payment had to be replaced by the bank due to fraud. Here's what will happen:
Approximately two weeks prior to the check in date, when the second payment is due, Airbnb will try to charge the credit card you originally used.
Once the payment is declined, Airbnb will send you an email to ask you to provide a different payment method with a link to enter the information.
Once you supply the new payment method info Airbnb will charge it for the second payment.
Don't worry...it is very easy!
Hope this helps!