Originally posted by loriandmatt
you can pay the difference between the price of any park hopper pass and the current AP price and upgrade. if you have already used any time on the park hopper pass, the AP will have an activation date of the first time you used your hopper.
This is true but only if you do the upgrade within the first seven days of use. After that, it's a straight case of them giving you a dollar credit for the remaining days of the old ticket and deducting that credit from the cost of your AP.
if you have not yet used the hopper pass, the AP will begin activation the first time you use it.
Almost. It will be active the moment you buy it. First use is no longer considered for expiration days. The year begins on the day you buy the pass at WDW or the day you exchange your voucher if you bought your AP elsewhere.