If you need to re-assign a credit card to a new user then set the card as expired and rename with something like old...... and then to add the credit card details to the new user as follows:
Click Settings, Users
Select the user which currently has the credit card assigned.
Scroll down to Credit cards
Click Actions/Edit
Select the Expired check box.
Change Card Holder Name
click OK and Save
Select the new user
Click Add credit card
Enter credit card details.
Click Save