You can, but I think you have to do it at the park, which is not fast pass friendly.
If that is indeed the case I would recommend buying a ticket to cover your trip, use it for your fast passes, then first thing at the park, upgrade the APs, then go into
MDE and assign your tickets to a 'fake guest' like Guest A. Then, when your passes expire you can use the purchased tickets toward the new APs.
Not 100 percent sure this is needed, but I think it is. Others can chime in.
Also, if this is what has to be done, try not to purchase the 'cross over' tickets with gift cards as that can cause a issue with reassigning tickets. (I bought a ticket for someone once who ended up not going, then tried to use that toward my AP and had to jump thru hoops since I had bought it with a gift card)