Don't change it. Just go and when you get to the podium, let them know two in the party will not be joining you after all. This happened to us on one trip. I took a friend with us and her cousin lived in the area so he was going to join us for one meal. But because of work he didn't know which night. So I made all our ADRs with counting him in. When we checked in, I just let them know that we had less people. It was never a problem. Its easier to do one or two less at the last minute, than add in one or two more.