If you are unable to get your dining assignment changed before you go, you can request it when you board. One of the first things you should do after boarding the ship is to seek out the maitre d' and ask him to change you to late seating. Unless the ship is totally full, he will probably be able to do that. Be sure to tip him at the end of the cruise if he is able to help you out!
The previous post has a good point, though - you may want to have dinner at the main seating on your first night. If you find you like your tablemates and that your digestion works better, you just might stick with it!