You don't necessarily pay the difference from what you PAID and the cost of the AP. This might be an extreme case (it happened with me), but if you got your ticket at a discounted price (I got a group discount through a corporate thing), then you pay the difference between the VALUE of the ticket and the cost of the AP.
What I mean is, I got a 5-day parkhopper (valued at $204) for $184. I upgraded to a Deluxe pass and I did NOT pay the difference between what I paid and the cost of the AP - I paid the difference between $289 and $204 ($85 paid).