I am trying to change the Credit Card used for my monthly iCloud charge

See the article If you need to change or update your Apple ID payment method - Apple Support

If you change your address, the credit or debit card information that you use with your Apple ID might be automatically updated by your financial institution, and you don't need to take action. But if there's an issue, add a new card first, then remove the old card.

Trying to change credit card used for monthly iCloud fees