Here's the faqs from
www.wdwig.com
How does WDW credit my old passes towards new ones?
Walt Disney World credits are based on what you *paid* for the ticket that you are exchanging. Example: If you have an unused one-day ticket that you paid $20 for then your credit will be $20 towards your ticket purchase.
For multiple days, the credit is based on the number of days of the ticket, the number of unused days and the total original cost of the ticket (though it has never been completely verified it is close to the actual formula of unused days divided by number of days times original cost.) Example: You have a 5 day Park Hopper pass you paid $200 for when you bought it. You have two unused days left on it. You would get a credit of 2/5 of the original purchase price. In this case, it would be $80. Note that Disney does not apply any dollar value to unused Plus options on Park Hopper Plus passes. Only unused major park admissions carry a cash value when upgrading.
Applying the credit from old passes can only be done at a Guest Relations location at one of the major parks or at either of the two Guest Relations offices at Downtown Disney or at Guest Services at your resort.
I have heard that I can upgrade my park hopper pass to an AP or PAP and get full credit for the original purchase price even after I have used a few days admission. Is this true?
Yes it is. If you decide within 7 days of first using a hopper pass that you want to upgrade it to an AP/PAP, you will get full credit for the price of the hopper pass towards the AP/PAP. The AP/PAP will be backdated to the date the hopper pass was first used. But beware: If you get a hopper plus pass and use one of the plus admissions before doing this, you will only be allowed to upgrade to a PAP.
Hope this helps.
It looks like if you are upgrading a hopper to another hopper then they will credit the original price, but as Goldenoldie said you will only get the full price of your hopper to AP if you make the switch within 7 days. Didn't realize that before.