Change my discover card number

Change, add, or remove Apple ID payment methods - Apple Support

maybe try adding the new card as a new payment method, then removing the old card after the new one is added

If you can't change your card number, probably you have pending payments on the old card.

In such a situation, I suggest that you contact iTunes Store Support Team directly, using this specific link:--> Apple Support (if you get no help, look at the bottom of the above link, where you can contact them using "chat" or "phone" options).

Regards

Giulio