Make a 100% cloned backup before you do anything, then you won't lose anything you can't recover.
Ways to get back to itunes 8:
1) Restore the computer to its pre-iTunes 9 state using the 100% complete clone backup you made just before you installed any new software you hadn't tested. 🙂
2) Drag itunes9 to the trash along with associated iTunes preference files in your user preferences folder in your personal library folder. You may, however, also find that your iTunes library may no longer work, having been irrevocably upgraded to iTunes 9. You then have to drag the library file to the trash but you will lose all your playlists, play counts, and ratings. Then reinstall iTunes8, open it and re-import your music.