You'll save the amount that is between the price you paid for the ticket and the gate cost of that exact ticket when you upgrade. That's because they "bridge" the cost so that your upgrade value is the current cost of the ticket, not just the price you paid.
When we upgraded from our tickets bought from
undercovertourist to
DVC APs, we saved $90 total (3 people upgrading), compared to just buying the DVC APs outright.