Here's the way it's SUPPOSED to work (based on what happened with EE for us this week):
-When a ride is down during your FP+ window for that ride, the FP+ is automatically converted to a floating FP+ that can be used any time of the same day at the same park, either for that attraction or a different one. At MK or AK, it can be used for just about anything at the same park (AK had a few limitations, but it was all things I wouldn't want to use a FP+ for anyway, like meeting Mickey and Minnie). At HS and Epcot, you may be limited to using it for attractions in the same tier (not sure).
-You can still get a 4th FP+ while you have the floating one on your account, so don't blow the floating one on something that has plenty of times open already! Save it for the ride that went down or use it on another top priority. DH and I did this at AK with no problems.
-Just like a 4th FP+, you can use it to re-ride something that you have already used a FP+ for. I confirmed this with multiple CM's at AK... they all said it was valid for "any attraction except x, y, and z" (which were named specifically and I don't remember what they were, but again, they were nothing I was upset about not being able to use them for lol)
-Important: If this happens, do NOT show up for another FP+ attraction too early and try to get in before your window. If you do, the system thinks you are using your floating pass and it disappears.
-Important: While you have a floating FP+ on your account, be very careful to only touch your MB to the sensor once when using other FP+'s. DH accidentally let his touch Mickeys for too long, and the system thought he was riding Kali River Rapids twice. They were able to fix it at a kiosk, but it's an unnecessary hassle if you're aware and are careful to start with.