'Do or do not, there is no try.'
-Yoda
https://www.facebook.com/help/286027304749263/
Changing Public Group Privacy
Public groups can change to private. If an admin schedules the privacy setting to change, they have 3 days to cancel if they’d like to.
Once a group changes from public to private, the change is permanent.
Changing Private Group Privacy
Private groups can't change to public to protect the privacy of group members.