function my_set_user_registered( $user_id, $user_meta ) {

    if ( ! empty( $user_meta['user_registered'] ) ) {

        $userdata = array(
            'ID'              => $user_id,
            'user_registered' => date( 'Y-m-d H:i:s', strtotime( $user_meta['user_registered'] ) ),
        );

        wp_insert_user( $userdata );

    }

}

add_action( 'wpf_user_updated', 'my_set_user_registered', 10, 2 );