create the event.xml on your module under the Vendor/Module/etc/event.xml and add the below code

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="checkout_submit_all_after">
        <observer name="checkout_order_place_after" instance="VendorModuleObserverPlaceAfter" />
    </event>
</config>

create the PlaceAfter.php on your module under the Vendor/Module/Observer/PlaceAfter.php and add the below code

<?php

namespace VendorModuleObserver;

use MagentoFrameworkEventObserverInterface;
use MagentoFrameworkEventObserver as EventObserver;
use PsrLogLoggerInterface;
use MagentoSalesApiOrderRepositoryInterface;


class PlaceAfter implements ObserverInterface
{

    protected $_customerSession;
    /**
     * @var OrderRepositoryInterface
     */
    protected $orderRepository;
    /**
     * @var LoggerInterface
     */
    protected $logger;
    /**
     * @var MagentoSalesModelOrderFactory
     */
    protected $orderModel;
    /**
     * @var MagentoSalesModelOrderEmailSenderOrderSender
     */
    protected $orderSender;

    /**
     * @param MagentoSalesModelOrderFactory $orderModel
     * @param MagentoSalesModelOrderEmailSenderOrderSender $orderSender
     * @param MagentoCheckoutModelSession $checkoutSession
     *
     * @codeCoverageIgnore
     */
    public function __construct(
        MagentoSalesModelOrderFactory $orderModel,
        LoggerInterface $logger,
        MagentoSalesModelOrderEmailSenderOrderSender $orderSender,
        MagentoCustomerModelSession $customerSession,
        OrderRepositoryInterface $orderRepository,
        MagentoSalesApiDataOrderInterface $orderData
    )
    {
        $this->logger = $logger;
        $this->orderModel = $orderModel;
        $this->orderSender = $orderSender;
        $this->_customerSession = $customerSession;
        $this->orderRepository = $orderRepository;
        $this->orderData = $orderData;
    }

    /**
     * @param MagentoFrameworkEventObserver $observer
     * @return void
     */
    public function execute(MagentoFrameworkEventObserver $observer)
    {
        $order = $observer->getEvent()->getOrder();
        $orderId = $order->getEntityId();
        $this->logger->info(print_r($orderId,true));
        $guestemail = "test@gmail.com";
        
        $comment = sprintf(
            __("%s customer order converted to : %s guest user"),
            $order->getCustomerEmail(),$guestemail
        );
        $order->addStatusHistoryComment($comment);
        $order->setCustomerId(null);
        $order->setCustomerIsGuest(1);
        $order->setCustomerEmail($guestemail);
        $this->orderRepository->save($order);

        $order->setCanSendNewEmailFlag(true);
        $order->save();
        $this->_customerSession->setForceOrderMailSentOnSuccess(true);
        $this->orderSender->send($order);
        
        return $this;
    }
}