The cost of the upgrade will the difference between what you paid for the WDW AP and $773.34 (the cost of the Premier AP, including tax). The cost of the upgrade will be due in full at the time of the upgrade. I saw that you're on the monthly payment plan, so it's not entirely clear to me if you will also need to pay that off at the time of the upgrade as well or if you can continue to make the monthly payments. You might want to check into that.
If you don't get any answer here, you might hop over to the WDW board and ask CheshireFigment about the monthly payment plan.
Also, it doesn't matter if you upgrade the pass at WDW or DLR. So if you're closer to WDW (and go there before heading to DLR), you might as well upgrade at WDW.