I added the extra day today. There was no penalty, but when the CM first checked her reply was that there was no availability. I knew there was, but I asked her to see if other resorts had availability. That's when she realized that, because we were using an expired code (
Free Dining offer, which has ended since we made the original booking), the computer was not allowing us to add a night. She put me on hold for a while so she could ask "guest services" for help. When she came back on, she had the approval/ability to do it. So we added an extra day and got the free dining for that day also. And we did not add another day to our park tickets since we'll be leaving in the morning. All of this was no problem after guest services assisted. Had we not dug a little deeper, the initial answer would have been "no availability." Thankfully, I had a very nice CM who wanted to help! I stayed on the line to provide some nice feedback via automated survey, which she said she would really appreciate.