Thanks for looking for the date modified criteria. Too bad there isn't one, but that certainly would have done the job if it had existed.

I'm not ready to abandon my labels yet, but I did just bite the bullet, globally remove the label and redo everything.

It turns out that when you delete a label from the labeling system, it removes that label from every email (good) which is a great way to undo all the labels of the same type.

But it also turns the "Do this:" behavior in the filtering system to a blank (bad). So for the filters I'd previously set up for medical emails, I had to edit the filters and re-add a newly-created "Medical" label for those. Didn't take long, but was annoying to have to do.

Then I did some searches for the names of some hospitals and doctors I remembered and labeled those emails too. So at this point I've mostly done what I needed to do, but I'm sure there are some older emails that I've missed. Hopefully in the future, searches will work to help me find what I need to find.

Thanks!