Hello moa243,
I understand you would like to change your Apple ID payment method, but are unable to do so due to your iCloud storage subscription. I'm happy to help you sort this out.
Although you would not be able to remove your current payment method completely since you have an active subscription, you should be able to add the second card using the steps here: Change, add, or remove Apple ID payment methods. Once you've done that, you can move the order of your payment methods so that the new card is at the top since payment methods are charged from top to bottom in this list. You can then remove your old credit card information completely if you plan to no longer use it.
Thanks for using Apple Support Communities.
Take care.