I've gotten answers both ways, Bravesfan.
Look at your reservation in My Disney Experience. If you have the "Change Reservation" link showing, you might be able to drop/add dates. Last year I had a 4-night reservation in
MDE that did NOT have that link next to it and I had to call to try to add one night that I saw was available (same campsite level) in the Reservation tool. The CM told me that there was NOTHING available because the system, in the background, had to cancel the ENTIRE 4-night reservation and look for a NEW 5-night one of the entire length. It was during a busy time and nothing was showing available for my 5 nights. I told her that was stupid/crazy and she got angry. I ended up telling her thanks for her time but don't change a thing.
What I did instead was book a separate one-night reservation to get the night I wanted then waited a week before I called back and got the two reservations merged into one.
Bottom line is you might/should be able to make a change but Disney has taken a dim view on people cancelling nights at the last minute before arrival leaving them with unfilled, non-revenue producing empty rooms/sites. So I hope you can make the changes you need when you need to.
Bama Ed