Last year when we reserved our hotel room on my Discover card something happened where they issued me a new one, and I thought I changed it everywhere but I guess I didn't. On our 4th day there they went to charge the card and it didn't work, but they didn't tell me right away. (The restaurant said everything was fine so I didn't know till I tried to make another purchase). Once I figured out what was wrong it was quick to sort out but if you don't want to deal with it while you're there I'd just let them know it's changed. With ADRs if you plan to show up I don't think it's a big deal (now that I think about it the old card was probably the card in the system for our ADRs). They won't cancel them, and they won't need that card unless you don't show up or cancel. If that does happen I'm sure they'll just contact you.