I think they're asking something different, what if you think of a brand new tag after you have already tagged things for years? It is potentially a lot of effort to retroactively apply it.
I have my version of a zettelkasten system, where I put a tag/ list of tags at the top of the note file (timestamp being the note name). Combining this with something like ripgrep allows me to search the note file contents by those tags, and keywords.
A tag I used, didn't seem like the right one for the content as it matured and grew over time.
This meant creating a tiny script which would take the name of the tag and the note name, in which it is being used and look at backlinks and forward links and rename the tag based on usage.