Is an MP3 file's FID number set in stone for the duration of that file's life on the player?

Yes. Once a track is on the player, its FID will never change.

A FID can be recycled -- if you delete a tune or playlist, that FID will be reused. Gaps in the FID number space are a bad thing -- they waste space in the database, so we reuse them.

This could cause a problem. If we delete the old FID, we'll only delete the files that we know about. Then, when we reuse the FID, it'll get associated with the (left over) wrong lyrics file.