I wanted to link a thread I thought I’d done about this, but can’t find it.
Let’s start with the basics, terminology wise.
You can book three initial FPs.
If you pre-book them, ie: in advance, they must be in the same park.
On the day, these can be made in multiple parks; they are still your initial three FPs.
If you are planning to park hop, then just book 1 or 2 FPs in the first park.
Then, as soon as you tap in to the first one, you can book the remainder of your initial FPs in another park.
Those can be another tier 1 ride in a second park, a tier 1 and a tier 2 in the second park, or even two tier 1 rides in two different parks.
If you already have three FPs, you must cancel those in order to make them in another park. You cannot modify.
You managed to get FoP on a day you really want to be at Epcot.
Tap into FoP, and whilst in line book TT and SE at Epcot.
Tap into FoP and book TT at Epcot and Alien Saucers at DHS (both tier 1).
OK, actually you booked FPs at AK, but change your mind after one ride.
Cancel the remaining FPs in AK.
Book two more elsewhere, as above.