Change 'Used for' contact type of custom fields
Jonathan, Welcome to CiviCRM stackexchange. 

Sometimes its not possible to change custom group's used for since the CiviCRM creates a separate table for this with a Foreign key. However its possible to update if the 'Used for' share the same table. 

In your case you want to make it visible for Individual subtype then its possible through UI by editing the custom group and then selecting the contact sub-type from the dropdown list.