class myUsers {
    static function init() {
        // Change the user's display name after insertion
        add_action( 'user_register', array( __CLASS__, 'change_display_name' ) );
    }

    static function change_display_name( $user_id ) {
        $info = get_userdata( $user_id );

        $args = array(
            'ID' => $user_id,
            'display_name' => $info->first_name . ' ' . $info->last_name
        );

        wp_update_user( $args ) ;
    }
}

myUsers::init();