The procedure for changing the settings in each direction is as follows.
From All-Ages to R-18
You can turn on [Posting R-18 contents] from the [Manage Creator's page].
From R-18 to All-Ages
1. Set all public posts to Archive
2. From the menu go to the [Manage Creator's page]
3. Turn off [Posting R-18 contents] button
4. Go to [Manage Posts], and republish private posts.