So, the disclaimer, right off the bat, is that you could lose your Library settings and information--playlists, play counts, ratings, etc.--though the process below should maintain everything. Of course, before doing anything like this, you should have a stable backup of all your important data. And now, for the process via ASD user "GangstaPenguin":
- Be sure to quit iTunes 9. It may also be best to quit other non-essential applications during this process though they shouldn't be affected.
- Delete iTunes 9 (drag from the Applications folder to the Trash bin).
- Navigate to username > Music > iTunes. Delete the "iTunes Library" file.
- Open the "Previous iTunes Libraries" folder and look for the backup of your old pre-9 library; it should be dated at about the time you installed iTunes 9. Move the file to username > Music > iTunes, and rename it "iTunes Library".
- Download iTunes 8.2.1. Run the installer as usual and you should be good to go, running iTunes 8.2.1. In the case that you have issues, you can always re-download the latest version of iTunes (currently 9.0.1) from Apple.