If you have a Costco Amex, do you plan to open another Amex account to keep your Amex benefits? It looks like from this article, Citi Visa will send you a new card that will replace your Costco Amex card.
They say this should be seemless and shouldn't affect your credit.
I think I might want to open a AMEX card because of the great benefits. Though, I have to see if there are any annual fees in a new AMEX. I did like not having to worry about annual fees because there were no fees if I renewed my Costco membership each year.