Very hard to just switch parks with FP+, unless you want to not use your fastpasses for the park you are in, and wait at the other park until the times have gone by so you can try to get same day ones.
We just got back from a trip that included both previous weekends.
During the week FP+ was wide open most of the days for many attractions.
I would book 4th, 5th, ... FP+ for a different park right as we hopped or the same park just as we tapped in our 3rd FP+.
On EE, RNRC, TOT, TSMM I was able to book multiple times in a row as we were queuing. I found this very strange because the SB times were 25-30 min in many cases.
Weekends were a different story. I was glad to have our early afternoon FPs booked.