Thanks to those who answered my question too! I have actually done this a few years ago, but my memory had failed me--I remembered some kind of restriction on the type of pass(but I think it was if you bought one of the WPFM, if you use the Water Park option there was some question as to whether you would have to upgrade to a PAP.
IF you have a WaterParksandMore ticket and you USE an entry to waterparksormore, you then must upgrade to a Premium AP. Not sure if it was still a question you had, but I figured I'd answer all the same.
Some are saying to avoid AK...AK is where we had a *perfect* upgrade experience. There's one person on the boards who had a really hard time upgrading a UCT ticket to a renewal AP, and she had that hard time one month after my perfect experience, and she had it at AK. So you just never know.
What I DO know is that you should write down the current, with tax, ticket price for the day you'll be upgrading. Take the current, with tax,
DVC AP price for the day you'll be upgrading.
Know, and write down, the difference. I wrote down the difference for each person, and the total. I brought that piece of paper with me (er, notebook).
That way, I could know EXACTLY if the CM was doing it right. During the transaction, I even said "and are you bridging the price to the current one?" and she smiled and said "yes". And the cost was exactly what it should have been.
Writing it down, IMO, is key. That way you don't have to rely on memory at all. When we upgraded it was very very cold. When you upgrade it will almost certainly be very very hot. Both will sap your brainpower and make it hard to think. So write it down and carry that with you.
If the CM gives you a different number, stop the transaction, walk away with your tickets, try again elsewhere.
The difficult upgrade situation I mentioned above resulted in the higher-ups at WDW and at UCT communicating and making SURE that the upgrade situations were the way we say they are on the Dis. The CMs helping her were totally wrong, and almost certainly know it. It's the procedure, the written procedure, to bridge the price (according to Cheshire_Figment who is a ticketing CM whose bosses know he gives us the scoop), and someone NOT doing it is doing it *wrong*.