My upgrade experience in Disney Springs was great. I bought
UT tickets in 2/16 before the price increase, so 2015 gate prices. I inquired about how much adding a day to 5 day PHs would be. It was $0. Yes, nothing owed! I expressed my incredulity and she said that since the DS "Welcome Center is a pre-arrival destination, we honor the discounts...unlike the parks." Her words. I said that I would stop asking questions, then!
I upgraded my 5 day PH to an AP for the amount I had figured.
It was a very easy transaction and the CM was efficient, even though she had to scan and type quite a bit to come to the final tallies. This was all before going to the parks. We didn't do that until the next day.