UPDATE - 7am, EST, it was available. I tried after midnight, no luck. I tried just after 6am and no luck. 7am, I was good to go.
I have to say, some times I wanted weren't available. I can only assume the throngs of others who beat me to it must have been holding my times. I got 10 or 20 minutes off from the ones I wanted. But now (1:30pm) I just went in and was able to change to the times I wanted, so all is well. It paid off to book the rides and then watch and change a little later in the day.
Oddly, even though I had one ride chosen as a 2-3, the next ride I was picking gave me choices that overlapped that hour window. I thought it wasn't supposed to do that. I didn't dare choose one, as I didn't want it to release my previous FPP, though. Now, in hindsight, I wonder if it would have given me both.
Anyway, thanks all for your advice.