Pretty simple...ride once standby and then use my fastpass...or if it's Everest or Tower of Terror...ride standby over and over again until there is a wait and then use my fastpass.
Unless this is the first 3 pre booked FP+ of the day or one of the tiered options i see no reason to change the FP. Just tap your band and while you board make a new FP.
You didn't need to book a "fourth" FP (at that time) as you could immediately change your current unneeded FP to another attraction and/or time.
However, if the park's FPs are "tiered," the immediate use of a third unneeded FP would enable the possibility of booking another "Tier 1" FP at that time.
That's kind of how we run our strategy. For a park like Epcot, all the rides that "need" FP+ are Tier 1. So we book and go through our 3 FP+ as soon as possible, so we can grab another Tier 1...if there are any. When we get in line for FP+ #2 of the day, we will then see if we can move #3 up so we can get it done and unlock #4.
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