Since your tickets were minted BEFORE Feb. 2017, you'd use the 2016 "Disney price" for their upgrade value.
Subtract that value from the current (at the time you upgrade) AP price.
I decided to assign those tickets to my youngest daughter and I bought an AP online from Disney. My youngest daughter, my granddaughter, and I are going in August as a pre-graduation trip for my youngest daughter. I just recently found out that the Orchestra at my youngest daughter's school will be playing at Disney over Memorial Day next year (of course right after my AP expires!). Families are responsible for paying their own way for this trip. Now I'm thinking of upgrading those 6 day base tickets to an AP for youngest daughter to use on her school trip next year.
So, going by your response, the upgrade value would be $378.08 (2016 "Disney price") - $1,119.00 (current AP price...ouch!) = $740.92. Is this right?
Worse comes to worse, I guess I could always do what you recommended to someone else in a prior post - use a ticket then upgrade it through
MDE to get the correct price.