What if I decide I want to change to a PAP after I already used my AP?
You can do this. It will be handled just like any other pass upgrade. WDW will credit you the remaining time on your AP and charge you the difference between that and the PAP price. The PAP will be prorated to expire when your AP would have. If you would rather have a brand new upgrade to expire one year from the upgrade you can do that too but you must specifically ask for it and pay the price difference.