You can set user permissions (forums) for certain usergroups, This will prevent certain usergroups from veiwing private forums.
You set this by their usergroup permissions so when you upgrade a user, be it manually or by post count or any other method then they will see it automatically.
This took me a long time setting up on my forum, best to do when you've got few forums as opposed to 50+ But once set its fine you shouldn't really have to change it again unless you add new ones.