If you bought your tickets before the price increase in August, or you got them from a discounter like
Undercover Tourist, just make sure to USE the tickets first before upgrading them. That way, you should get credit for the full then-current gate price of the entire ticket.
Otherwise, you'll just get credit for what you paid, or if from a discounter, what the
discounter paid Disney for the tickets (less than you paid).
Not all Guest Relations folks know how to "bridge" the price so you get the full credit. You should have the numbers with you so you can correct them if they do it wrong. Take the
December price of the ticket you have, and subtract it from the cost of the AP ($499 pre-tax, $531.44 with tax), so you'll know exactly what you should pay. If necessary, ask for a manager.
Have fun!