How can we improve WHMCS?

I noticed recently that if the customer has an invoice on their account that is unpaid but not due yet, and they change their default credit card, that unpaid invoice will not be charged to the new default card automatically, instead it will be charged to the previous default card, unless it is processed manually.
WHMCS support explained to me that this is because when an invoice is generated it is automatically tied to the default card that existed at the time of the invoice generation.
I find this extremely unfriendly as I believe it is natural to expect that when you change your default credit card, all payments going forward will be charged to the new default!