Sorry, i don't know if it is possible. I haven't tried/tested private community on Piefed. I recall you can add username to community. However i wanted to react on switching private community to public. There are case where you want to do that.

However, i think it is sensible and shouldn't be allowed or not decided by the mods team at least. I wouldn't recommend switching private to public is the software. Because there are thing we talk between ourselve, and they should stay that way.

Maybe, it would require to delete the whole posts and comments history. Dunno, for me it requipe people approval. Or something like mastodon that hide your post from people that aren't in your friend circle and then everyone decide for themselve whatever is the community decision.

From a technical POV it is theoretically possible but not tested.

Most likely the various softwares would opt out of handling non-public content out of safety.

It is something we'd want to work on in lockstep with all the various devs 🙂