Ok here's my story - in September I had discounted tickets that I wanted to add an extra day to. I was told on the phone with ticketing that it could be done at the Guest Services either at DS or in any of the parks. They advised that this would be no problem and we even worked out the math for what the difference would be (losing the CDN discount and adding the extra days). As I got the ticket from CAA, Guest Services tried to tell me the only upgrade I would be able to do was to an annual pass. To the tune of like $600 - needless to say this resulted in tears that I'm not proud of. I stayed polite but I was really frustrated as I felt like the vacation we'd extended by 3 days was a total waste of time and effort. They had to get a manager to sign off on it, but eventually they were able to work it out so that I could pay the difference for the tickets to change them from being Canadian Resident tickets and then again the difference for the extra days and not have to get an AP at that time. It took at least a half hour to do as they had to over ride this thing and that. It was a mess, but they did sort it out in the end.
The moral of the story is, for only one extra day, plan a Disney Springs day or a waterpark day and save yourself the headache/heartache on the off chance that you'll have the same experience.