I noticed that the website was not showing availability for us but they did let me add a day anyways. A week later I tried to remove and they would not let me remove it. Which was ok, we will just stay but I noticed they are much more likely to add than remove. I didn't think they would let me add the day, because it was not available to book as a new reservation online with those dates, but they did.
I also add a day to our trip too, but it's at the end, however, I will have to change the room, "water view" instead of "standard" because they said the standard room is not available for my stay. In the end, I paid about $500 more for that extra night but it is kind of a wash since the plane ticket is cheaper if leaving a day later. I guess it depends on the CM, I noticed some are better at their job then some others.