The day you board, go to wherever dining changes are being made. It will be listed in the Navigator. For example, on the Magic and Wonder it's likely to be Lumiere's or Triton's (respectively). You should be able to go there and see that your request was met and if not, they will be able to see what they can do. We've made dining changes before and it's relatively easy, especially if you're on late dining. You didn't say which ship or dining time so it's hard to say. But look in the Navigator on the front page, usually the lower left column it will tell you where to go to make those changes.