Hi. Hum... not sure if it is possible...
In
this topic that is still from this year, Lapo says that in "the next major update" it will be possible.
So... there's not yet a method to change a room from private to public.
But, you can always do it your way instead using sfs way hehe.
For example, you could create a room variable called "private" and then you could change it as you want (its recommended for you to set the variable owner to null, set persistent to true and only update it via a server-side extension).
Then on the client instead of doing if(room.isPrivate()), you would do if(room.getVariable("private"))...