Thought about this for a while, we should probably create a ticket for community to add, but in my mind we’d only offer public channel to private group.
Current thinking is the damage that could be caused by turning a private group into a public channel is massive compared to the limited administrative benefit it offers.
Also, per above. a System Admin could do it in the database (and perhaps we add a CLI tool option for this in future).
Having the option in the user interface doesn’t seem worth the risk–it’s really difficult to review content might have been shared in an old or busy private group.