Try creating a preset using a single image to reverse the last change. Uncheck all boxes that don’t apply. Then select all the images and apply the new preset.
The other option is to revert your catalog to an earlier backup and then apply the intended adjustments.
Not sure how to do a preset that just backs the history off one step. Since many images have different processing already there isn't just a couple settings I can change to reverse the effect.
I had done a bunch of processing so rolling back to a catalog backup would loose all that.
OK, I was able to sort out my fewer processed images to other folders and all the remaining had only the undesired sync in their history. I then selected them all in Library and did photo/develop settings/reset.
This did what I needed.
I saw there is another old thread with a script that undo's history one step, but comments indicate this doesn't work with newer LR versions and in fact messes them up instead. How do I to mass-delete develop preset from selection of images?
It might be cool if some SQL person updated this script to work again, though hopefully I won't need it again.