Our bank is concerned that our account may have been hacked, so we have had to change our credit card number. I know that our B&N books are somehow linked to our credit card number. Does the old number stick to the old books (so we shold keep a record of the number) or does it switch over to the new number?
The old number will be the DRM key to the already-downloaded books, so it's a good idea to keep it around. Books that you have not downloaded yet, or not purchased yet, will download with the new number once it's added to the account and marked as default.
if you want a book you've downloaded to be associated to the new number, just archive and unarchive it.