To expand what @AuntB_luvsDisney said, you can have three initial fastpasses. If you book these in advance you can only do so at one park. You can book one, two or three of your initial fastpasses in advance.
If you book less than three, then once the FP+ system knows you’re in a park, i.e.: once you tap into FP number one, you can then book the remainder of your initial three fastpasses in any other park(s), at any tier (no restrictions).
If you have booked three in advance, and change your mind after doing the first one, you can then CANCEL the rest of them and book them in another park without restrictions.
This does not work in the same park!
Some examples:
- You manage to get FoP on your last park day. But really you want to be at MK.
- after tapping into FoP, you go onto MDE and book Space and Pirates for later on.
- You book FEA and Figment at Epcot. Then plan to go to DHS.
- as soon as you tap into FEA you can look for TSMM or RnR or ToT and book one. You don’t have to wait until you’ve ridden Figment.
- You book FoP, Everest and Dinosaur at AK. Having ridden FoP you decide that you’d rather hop over to Epcot.
- you then CANCEL Everest and Dinosaur and book Soarin.
- you then notice you can get TSMM in the evening, so you can book that too!
You’ll notice the word CANCEL is in bold and capitals. That is because you must cancel the fastpasses in question, not modify. Modify will not work for park hopping!
And again, this does not work for the same park.