Once you pass through a turnstile once with the tickets, when you go to upgrade them they should be worth the current-that-day cost of that ticket. So then you just subtract that from the total cost of what you're upgrading to.
Now, this is dependent on the CM "bridging" the tickets you have to that day's price. So you want to know the cost of the upgrade (per ticket and total...when I did this, I actually wrote it all down because math when I'm at a themepark is impossible). If they give you a higher price, ask if they've bridged the tickets. If they didn't and won't, just take the tickets back and try again later or at a different place. A dis veteran just yesterday or so had to keep pushing the CM to check with her supervisor. 3 different times the CM had to check because she kept saying "no" when the answer was absolutely "yes", the CM just didn't realize it.