When we converted our PH's to AP's...............
--they scanned the ticket to figure out how many days you had remaining on the pass
--I think it also told when the ticket was purchased & what you paid for it at that time
--Then they figured out how much each day was worth (ie - you had a 5 day pass that you paid $200 for but only used 2 days, the remaining days could be worth $40 each)
--The "remaining" amount of money is then applied to the AP
--unused water park days have no monetary value so if you have a PHP w/ water park days on it you may want to consider whether it's a better value to keep the pass & use the water park days first before doing an upgrade or doing the upgrade & losing the WP days.
We didn't have any problem upgrading. We did it right at the TTC. Good luck.