As Aladora said, if you want to use the MM you will need wait and upgrade afterwards. I just wanted to throw out another aspect to consider though. While buying tickets and upgrading at the park has the advantage of allowing you to use your MM before upgrading, buying an AP from the start has its advantages too. The main one in my opinion is that if you purchase your AP ahead of time and then the price goes up before your trip, you will not have to pay the new higher price. Whereas if you purchase tickets and then upgrade to an AP when you're there, you'll be paying whatever the current price is when you're there. AP prices seem to go up about once per year to varying degrees, and the closer we get to Star Wars land opening the more I would expect them to increase. Also, you would be able to use AP discounts from day one.
Since you're not going until December 2018, it's pretty likely that there will be a price increase at some point before then. I believe the vouchers are good for any time in the same calendar year (I would double check this though), so you could purchase your AP's in January 2018 to lock in the price, and not activate them until December.