Found this helpful when working with my collection: How to update/rescan your iTunes library if you have:
– Added new songs: This was one of the most frustrating thing about iTunes. I would add a few songs to a folder which had already been added to iTunes but there was no way to tell iTunes to rescan that folder and pick only the updated songs. Every other jukebox software I know has this feature. Winamp even lets you schedule when you want to rescan the folder! What did I know. This is actually simpler in iTunes. Just add the folder again to iTunes. If you have not moved the existing files from their previous location, have not renamed them, or done anything else to them, they would be ignored and only the new files would be added. All the old songs would maintain their playcount and rating etc. I regularly point iTunes to my top-level song folder and they get added just fine.
-Updated tags outside iTunes: I use GodFather to edit ID3 tags for my mp3s but since iTunes maintains its own database, the changes are not visible in iTunes. You can clean your iTunes library and add those songs again but you’d lose playcount and rating. The correct way to make those changes visible in iTunes is to select all files (using ‘Ctrl-A’), click on ‘Get Info’, and click ‘Ok’. iTunes will pick up all the tag changes without losing playcounts or ratings. Do NOT check any of the checkboxes for any of the tag fields. You’d nuke all your tags. Just click ‘Ok’ and it’d be done.