What you want to do is copy the Registered Users group and make that new group the Promoted one. Copy both the usergroup permissions and the forum permissions for the group. Then you can create a Promotion to move users from the Registered group to the new Promoted group after x posts. You may rename the Registered usersgroup to the Newbie group if you want. Then run the Promotion manually through the Scheduled Tasks and it will move all the users for you. Now you can change the permissions for the Newbie (old Regsitered) group to have the more limited permissions.