change user role after changing order status on woocommerce

add_action( 'woocommerce_order_status_changed', 'your_function', 99, 4 );
function your_function( $order_id, $old_status, $new_status, $order ){
    if( $new_status == "completed" ) {
		var_dump($order);
        $user_id = $order->get_user_id();
		$user = new WP_User($user_id);
		$user->set_role('subscriber');
    }
}