Lots of threads on this. After reading my reply do a search.
Simply go to customer service desk on board and NICELY ask. Thats it. You might ask again late the day you board.
You really cannot tell if there are vacancies because last minute cancellations are possible, but would c likely be filled via MoveUp 48 hours before.
If there are no shows and they have not contacted Celebrity with travel delays, an upgrade is possible. Check out the MoveUp page so you could have an idea of the $ and if you are even interested.
They happen rarely on a ship 90% full. But possible. Good luck.