We've always bought a package (3 day tickets, get an extra day free) deal, and then upgraded to the APs at the end of our trip. Sometimes the package deals have some fun benefits (free trading pins, etc), and this way we could start our trips during blackout times. Normally I purchase the Premium, and the rest of my family have the deluxe (to maximize the discounts on meals and purchases).
I know when we considered upgrading mid-trip it was during a blackout time, and we were told that we would have to pay the upgrade fee the following day if we came in the park, but I don't remember them saying anything about the current day. That may have been since we were already inside the park with our park-hopper tickets. I'm sure someone here will know for certain.