I wrote a post, tagging two people, intending it for their eyes only. A good conversation was had, but a week later, I noticed getting +1s on it from other people I didn't intend to share it with. I'd like to keep the post, its comments, timestamps et c, both for the tagged people's and my own benefit, but drop its most accidental "Public" status. (I don't mind keeping the +1s, too.)
This is more or less the opposite question to How to add user to an existing post in Google Plus? – and I can't think of any good reason why I would be forced to either kill it for everyone, or keep it available for everyone. Every day it sits openly accessible, it keeps randomly leaking to people it wasn't meant for, eroding away the privacy the original post was meant to have.