Even if you don't get it, stop by the dining room after you board and ask for a switch.
On our cruise last August, our reservation was for the late seating (said so right on our tickets). But, for some reason, there was info in the room w/ a seat assignment for the early seating. We did not discover this until more than an hour after the early seating had started. The cruise was full, and I was not optimistic and a little upset at the prospect of missing our first night at Lumiere's.
We went straight for the dining room, and one of the head waiters told us to come back for the late seating and he'd see what he could do. Sure enough, there was a table for us!
I suspect that there is all kinds of shuffling going on throughout the cruise, and if you persist, you'll likely get what you want, even if the wait list doesn't come through.