I don't know if this will work with your discount tickets, or if it even works at all anymore, however, you used to be able to "lock in" the full face value of the tickets by using them.
Lets say you paid $300 for what are face value $400 tickets. If you took those tickets directly to guest services and tried up upgrade them, you would be given $300 for each ticket - what you paid.
However, if you were to use a ticket to go into a park, and then go to guest services, the act of using the ticket causes it to "become" a regular ticket, and you would then get $400 towards your upgrade (you can upgrade a ticket at any time up to the last day of use)
I did this with tickets that I got through work at a discount. I upgraded them to an AP and got full face value, not the tickets at work discounted price.
I did it about 3 years ago, so things may have changed, but I have not heard otherwise.
-dave