hiroMYhero is correct. This is exactly what we did last year. We bought a 10 day pass with parkhopper and water park. We initially had no intention of getting an AP. Our trip was planned for 16 days. We were going to do 10 park days and 8 or 9 water park days (we were going to have some days where we did both, but some days where we did water park only). About 4 days into it, we were floating around the lazy river at BB and just shooting the breeze. We got to talking about coming back to WDW, so we decided to look into the AP. After asking a few questions, we decided to upgrade. It didn't effect our FP+ plans at all, plus they refunded us for all of the parking we had paid for before our upgrade (we kept the receipts, luckily). They even gave us our discount on merchandise we had already purchased...again, we had the receipts.