To re-assign a credit card to a new user, first expire and rename the card on the old user's record, then add the card details to the new user as follows.
Click Settings, then click Users.
Click the user who currently has the credit card assigned.
Scroll down to Credit cards.
Click Actions, then click Edit.
Select the Expired checkbox.
Change the Card Holder Name to indicate it is no longer active (for example, add '-OLD').
Click OK, then click Save.
Click the new user.
Click Add credit card.
Enter the credit card details.
Click Save.