users cannot make public branches private even if they have an appropriate commercial subscription

[Martin Pool]

If you create a public branch, there is no way to make it private (except perhaps by losa intervention.)

It would probably save trouble if people made private projects private from the start (see bug 750871) but sometimes they do end up with a branch that should be private and is actually public. It would be good to be able to escape from this.

This needs to check whether the project's policy and entitlements allow branches to be private. If not, ideally it would explain why it can't be done.

There is an issue that if you make a branch private that other people have stacked upon, you may break their branches. However, in this case that the branch was actually meant to be private, this shouldn't be a a blocker: the other branches have no business being private; and if there's no direct means to set privacy users will have to delete the branch and create a new one which will also break dependent branches.