I was just editing an older post, and I realized that the changes I were making were unprotected, as I saw no way to save them in draft form. I wasn’t ready to commit the changes to the public (Update Post function), but I did want to save them to disk, so I could work on them later.
Unless I’m missing an obvious button on the edit page, I think this would be a worthwhile improvement.
You can temporarily set your post to draft again. Or set the post to private. Then you can edit and update without having half-finished versions live online.
Another alternative, especially if your editing will take a long time, is to copy the html of the post you want to edit into a new draft post. Do all the editing you like, then copy the html of the edited post back into the published post, replacing all the old code with the new code.