I don't know how long you're going for, but if it's longer than the ticket you're thinking of buying and then upgrading, I recommend not doing that.
I bought APs that we will activate when we arrive on Aug 17. When my 60 day window opened, I had no problem making all of my FP ressies for the duration of our two week trip.
If you buy a regular ticket with the intention of upgrading, you might save a couple of dollars, but the number of FPs you'll be allowed to book at the 60 day window will be limited to the number of days your ticket is for....