Someone recently contacted me with this question and I figured I’d share it with everyone. They found me from a search from organizing checked items for iTunes.
What I am about to show you how to do isn’t very new, but it is a trick that might come in handy in cases that require desperate measures. Use with extreme caution. Practice this in a small playlist that isn’t mixed with checked and unchecked tracks. If you hold [Control/Command] and click on the checkbox next to any track, it will change all files to the opposite check mark. And for real novices that don’t know how to edit more than one track at a time, but don’t want every single file, you can select multiple tracks in sequence using shift or you can highlight other tracks using CTRL/CMD (but not on the check box itself).