It's a glitch that a small number of people were able to do. No telling if they still can or if it's patched. I tried when I was there to see if any loophole would work through my phone and I got nothing.
You can cancel the 3 FP's before you use them and repick all over. But if you use one, unless the rare glitch happens, you can only swap around rides within the same park. And then once all 3 are used, only a kiosk as of right now is how to add the additional ones