I seem to remember that there is an issue with overwriting FIDs directly in that the empeg stores an offset into the MP3 file as to where the audio data starts. If you change the ID3v2 tag's size, then this information becomes incorrect, potentially affecting playback. Now, I never got a confirmation that this is how it works, but it seemed to follow from some previous statements.

Anyone that would like to say I'm wrong, please do so.