How to replace an MP3 or M4A in iTunes with a higher quality file, without having to re-do playlists containing the track?
You can just replace the file in the filesystem.

Unfortunately, iTunes is too clever for it's own boots and registers for Finder events. So renaming a file in Finder will update it's location in iTunes if iTunes is open!

So here's one way I've tried that works.


Quit iTunes (actually quit, not just close window);
Locate the file in Finder:

This could be done in iTunes (before quitting) by right-clicking and choosing Show in Finder;

Replace the file in Finder with one with the exact same filename;
Open iTunes


iTunes will then use this file when you attempt to play it. Note: if any of the ID3 tags differ they will be updated in iTunes too (which is probably what you want). This means if you replace it with a file with the same file name but with less information in the metadata, you'll have less information in the iTunes browser.