Dave is right, do it ASAP. You shouldn't have a problem; it's harder to get changed from second seating to first, than from first seating to the second. If you go to the dining room around 1pm you will see people in line to make different arrangements also. One of the first things we do when we board the ship is to find our table. If it isn't what we asked for, we get in line to make the necessary change.