bbPress.org

ok, let’s try setting both

function change_display_name( $user_id ) {
    $info = get_userdata( $user_id );
	$args = array(
		'ID' => $user_id,
		'display_name' => $info->user_login 	);
'nickname' => $info->user_login 	);

    wp_update_user( $args );
}
add_action('user_register','change_display_name');

This should set both display_name and nickname to the username