yes you can, they have a desk set up the day you embark to be put on a "waitlist' to change assignments. I would think it would be a lot easier to get changed to late, rather then vice-versa.
Also, a friend of mine who cruised in early September wanted to be changed to early from the late seating. When she went to go and change they told her early was full. The decided to get dressed for the early seating anyway, and stop by the dining room and ask the dining room manager at the door if it was possible.
Just like that.. the got early seating. Not saying it's always possible, but sure worth to try!