According to reddit though, it isn't as cut and dried as installing the new version (12.6.3; yes, it's a little confusing) and downloading apps. "Your iTunes library will still not be compatible after installing iTunes 12.6.3 on top of iTunes 12.7. You either need to revert to a backed up copy of your previous library, or you need to open iTunes while holding down the Option key (or Shift key on a Windows PC) on your keyboard, then choose 'create a new library.'"

This new-old version of the software should work with any official ringtones you've snagged, audiobooks and iTunes U downloads. Now that this is out of the way, maybe Apple can get back to fully debugging iOS 11.