While I can't offer you a definitive answer, here is what happened to us during the FP+ testing last year:
We had a FP+ for TSMM, but the ride broke down while we were in line. Within minutes, I received an email stating that I could choose to FP any other ride in the park that day (at any time), or I could FP TSMM if the ride became available again that day. If we chose to FP another ride, the FP for TSMM was gone. We ended up using the FP on RnRC that day. All we had to do was scan our bands for it to work.
I would think that something like this is still the case, but I'm not absolutely sure.