I want to modify/see the list of shared circles/users of my old posts but I can’t find it anywhere.
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That's intentional. Imagine you shared something privately with me, then i commented on it with something that is not meant to be public, thinking that only you and i can see it, then you change the visibility to public and i'm screwed :D.
If you really want to reuse the old post, then you can reshare it with a different visibility. Reshare doesn't tag along the comments, so that's safe.