We just went to WDW earlier this month. We had previously bought our tickets through Sam's Club and when we called MS, they just charged us the difference for the Sorcerer Pass. You should be able to call and upgrade even though you have tickets. My parents did the same thing earlier this year.
We were able to make all of our park reservations for our trip earlier this month with our APs loaded on the
MDE.
When we went ealier this month, our APs were already loaded on our accounts when we arrived at GS at MK our first day (two adults and three children). GS told us to just go to the AP line and they would be activated upon scanning in the first time.