@macmanx: Thank you for your reply!
I feel that my question applies to many users. So, rather that filing a private request, I'd like to give more details:
What prompted my question is that my domain name (registered elsewhere) came up for renewal last week. My domain name could not automatically renew because my credit card had expired. I just logged in and updated the expiration year.
My WordPress.com upgrades (domain, no ads and css) do not expire until November, but rather than wait until the last minute I wanted to ensure that my automatic renewals went seamlessly.
On the "Upgrades --> Domains" page there is a button that says: "Update Credit Card". When I tried to put in just the new expiration date all the blanks turn red and it apparently wants me to fill in everything from scratch.
Everybody with a credit card has an expiration year (I am assuming) and a new card is sent out. Since WordPress.com upgrades are set up to automatically renew, it seems that WordPress would want it quick and easy to revise just the expiration year if everything else is the same. (After all, that button says "Update Credit Card", not fill in all credit card info from scratch".)
I request that the Credit Card Update page be revised for the convenience of all of us.