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.