This is definitely a lifesaver...
I am restructuring my forum and have moved multiple 1st level tag into 2nd level tags.
In order to grant the new parent to all the old tags, i simply search:
tag:2nd-level -tag:1st-level
to locate all the posts not yet assigned with the new 1st level tag
then with @clarkwinkelmann's tool I am able to swiftly update all the posts! you are amazing!