It depends on where you bought the ticket from. If you bought it from a reseller, you should have paid less than gate price. (Otherwise why use a reseller?) Using it to enter the park once locks the value of the ticket at the current gate price. You would then need to go ask guest services to "bridge" the ticket. They should this automatically, but some CMs don't. Bridging the ticket means they will apply that current gate price towards the cost of your upgraded ticket. If they don't bridge it properly, or you don't use it to enter the park at least once before doing the upgrade, you will only be credited what the reseller paid WDW for the ticket. This means you would be credited less than you actually paid for it. In some instances, buying the ticket direct from WDW or as part of a package won't entitle you to bridging (I can never remember if it is tickets from a package or ones bought from WDW direct).
I believe a 7day hopper is currently $467.54. So say you want to upgrade to a Platinum pass at $797.69. Using that ticket at least once means it should now have a value of $467.54, so applying it towards the upgrade means you should have to pay an additional $300.15 per person. If you bought it from a reseller and say paid $450 for that ticket, you would save $17.54 on the cost of the AP. But if you bought it from a reseller and didn't use it once before upgrading, WDW might only credit you $425 (making up a number here) as that is what they charged the reseller.