Edits on Amethyst implement this NIP. It allows users to replace the content of their posts but not the extra metadata - they can't move an existing post to a new thread, for instance.

The entire history is kept. Clients can show the history, starting with the latest edit.

Other users can propose edits by sending it back to the original author. The author accepts by creating a new edit by themselves.
 https://github.com/nostr-protocol/nips/pull/1090 

 oh I didn't know about the suggested edits feature. that opens up interesting new avenues....