Also, this is all enabled by the PPSLOCKED bit in the PPSLOCK register, which defaults to 0 (unlocked).
To set/clear PPSLOCKED you have to follow the sequence shown in sect 12-4 exactly.
There is also a CONFIG PPS1WAY setting in CONFIG2 which, if set, only allows you to change it once.

I'd recommend setting CONFIG PPS1WAY = OFF and not setting the PPSLOCKED bit. That way you don't have to bother with it.