It just depends upon room availability.
1) If there are rooms available before the day of the cruise, you can call and pay to move to another room. I suppose that you wouldn't have to pay if you moved in the same category (again, based upon availability).
2) If there is availability on embarkation day, you will be able to pay (a lesser amount than if you "upgraded" days earlier) to upgrade. Generally speaking there are very few rooms available to move to on sail day.
3) Go and enjoy the cruise anyway. None of the rooms are that horible. Remember, any room on the ship - where they are cleaning it for you, serving you wonderful meals (again - NO cleanup), and no job to go to HAS to be better than being at home, at work, cooking meals and cleaning up.