Magento 2: Replace old email variable with new ones (Files and Database) 2.4.3 to higher. https://developer.adobe.com/commerce/frontend-core/guide/templates/email-migration/. Place this file in MAGENTO_ROOT/scripts.

findAndReplaceEmailVariables.php

<?php

$time_start = microtime(true);

// MAGENTO START
require dirname(__FILE__) . '/../app/bootstrap.php';

use Magento\Framework\App\Bootstrap;

$bootstrap = Bootstrap::create(BP, $_SERVER);
$objectManager = $bootstrap->getObjectManager();

/** @var \Psr\Log\LoggerInterface $logger */
$logger = $objectManager->get(\Psr\Log\LoggerInterface::class);

/** @var Magento\Email\Model\ResourceModel\Template\CollectionFactory $templateFactory */
$templateFactory = $objectManager->get(\Magento\Email\Model\ResourceModel\Template\CollectionFactory::class);
$templateFactory = $templateFactory->create();
$templateCollection = $templateFactory->load();

/**
 * OPTIONS:
 * --dry-run: runs the script without executing this.
 * --files: runs the script for files
 * --database: runs the script for database files
 **/

try {
    // CLI Options:
    $dryRun = false;
    $runForFiles = false;
    $runForDatabase = false;

    // Set options by CLI variables
    if (count($argv) > 0) {
        foreach ($argv as $argument) {
            if ($argument == '--dry-run') {
                $dryRun = true;
            }

            if ($argument == '--files') {
                $runForFiles = true;
            }

            if ($argument == '--database') {
                $runForDatabase = true;
            }
        }
    }

    // Key: before value, value: new value
    $variableReplaces = [
        // General variable:
        'htmlescape' => '',
        'var=$' => 'var ',

        // Variable openers:
        '{{ ' => '{{',
        ' }}' => '}}',

        // Store variables:
        'store.getFrontendName()' => 'store.frontend_name',

        // Order variables:
        'order.getCreatedAt()' => 'created_at_formatted',
        'order.getCreatedAtFormatted()' => 'created_at_formatted',
        'order.getCreatedAtFormatted(1)' => 'created_at_formatted',
        'order.getCreatedAtFormatted(2)' => 'created_at_formatted',
        'order.getCreatedAtFormatted(3)' => 'created_at_formatted',
        'order.getIncrementId()' => 'order.increment_id',
        'order.getGrandTotal()' => 'order.grand_total',
        'order.getOrderCurrencyCode()' => 'order.order_currency_code',
        'order.getBillingAddress().getName()' => 'order_data.customer_name',
        'order.getShippingAddress().getName()' => 'order_data.customer_name',
        'order.getCustomerName()' => 'order_data.customer_name',
        'order.getEmailCustomerNote()' => 'order_data.email_customer_note',
        'order.getIsNotVirtual()' => 'order_data.is_not_virtual',
        'order.getShippingDescription()' => 'order.shipping_description',
        'order.getFrontendStatusLabel()' => 'order_data.frontend_status_label',
        'order.getStatusLabel()' => 'order_data.frontend_status_label',
        'order.getPayment().getMethodInstance().getTitle()' => 'payment_html|raw',

        // Invoice variables:
        'invoice.getCreatedAt()' => 'created_at_formatted',
        'invoice.getCreatedAtFormatted()' => 'created_at_formatted',
        'invoice.getCreatedAtFormatted(1)' => 'created_at_formatted',
        'invoice.getCreatedAtFormatted(2)' => 'created_at_formatted',
        'invoice.getCreatedAtFormatted(3)' => 'created_at_formatted',
        'invoice.getIncrementId()' => 'order.increment_id',
        'invoice.getGrandTotal()' => 'order.grand_total',
        'invoice.getOrderCurrencyCode()' => 'order.order_currency_code',
        'invoice.getBillingAddress().getName()' => 'order_data.customer_name',
        'invoice.getShippingAddress().getName()' => 'order_data.customer_name',
        'invoice.getCustomerName()' => 'order_data.customer_name',
        'invoice.getEmailCustomerNote()' => 'order_data.email_customer_note',
        'invoice.getIsNotVirtual()' => 'order_data.is_not_virtual',
        'invoice.getShippingDescription()' => 'order.shipping_description',
        'invoice.getFrontendStatusLabel()' => 'order_data.frontend_status_label',
        'invoice.getStatusLabel()' => 'order_data.frontend_status_label',
        'invoice.getPayment().getMethodInstance().getTitle()' => 'payment_html|raw',
        'billing.getName()' => 'order_data.customer_name',
        'billing.name' => 'order_data.customer_name',

        // Handles:
        '{{layout handle="sales_email_order_creditmemo_items" creditmemo=$creditmemo order=$order}}' => '{{layout handle="sales_email_order_creditmemo_items" creditmemo_id=$creditmemo_id order_id=$order_id}}',
        '{{layout area="frontend" handle="sales_email_order_invoice_items" invoice=$invoice order=$order}}' => '{{layout area="frontend" handle="sales_email_order_invoice_items" invoice_id=$invoice_id order_id=$order_id}}',
        '{{layout handle="sales_email_order_invoice_items" invoice=$invoice order=$order}}' => '{{layout area="frontend" handle="sales_email_order_invoice_items" invoice_id=$invoice_id order_id=$order_id}}',
        '{{layout handle="sales_email_order_items" order=$order}}' => '{{layout handle="sales_email_order_items" order_id=$order_id area="frontend"}}',
        '{{layout handle="sales_email_order_items" order=$order area="frontend"}}' => '{{layout handle="sales_email_order_items" order_id=$order_id area="frontend"}}',
        '{{layout handle="sales_email_order_shipment_items" shipment=$shipment order=$order}}' => '{{layout handle="sales_email_order_shipment_items" shipment_id=$shipment_id order_id=$order_id}}',
        '{{layout handle="sales_email_order_shipment_track" shipment=$shipment order=$order}}' => '{{layout handle="sales_email_order_shipment_track" shipment_id=$shipment_id order_id=$order_id}}',
        '{{block class=\'Magento\\\\Framework\\\\View\\\\Element\\\\Template\' area=\'frontend\' template=\'Magento_Sales::email/shipment/track.phtml\' shipment=$shipment order=$order}}' => '{{layout 
handle="sales_email_order_shipment_track" shipment_id=$shipment_id order_id=$order_id}}',

        // Amasty Gift Code emails
        'gcard_email.getGiftCode()' => 'gcard_email.gift_code',
        'gcard_email.getExpiryDays()' => 'gcard_email.expiry_days',
        'gcard_email.getRecipientName()' => 'gcard_email.recipient_name',
        'gcard_email.getSenderName()' => 'gcard_email.sender_name',
        'gcard_email.getSenderEmail()' => 'gcard_email.sender_email',
        'gcard_email.getSenderMessage()' => 'gcard_email.sender_message',
        'gcard_email.getBalance()' => 'gcard_email.balance',
        'gcard_email.getExpiredDate()' => 'gcard_email.expired_date',
        'gcard_email.getImage()' => 'gcard_email.image',

        // Magetrend variables
        "mtVar.getData('invoice_increment_id')" => "invoice.increment_id",
        "mtVar.getData('invoice_increment_id')" => "invoice.increment_id",
        "mtVar.getData( 'formatted_grand_total')" => 'mtVar.getFormattedGrandTotal()',
        "mtVar.getData('formatted_grand_total')" => 'mtVar.getFormattedGrandTotal()'
    ];

    $replacedArray = [];
    $replaceFileItems = 0;
    $replaceDbItems = 0;

    // Gather all emails for code + theme
    $frontendDir = BP . '/app/design/frontend';
    $appCodeDir = BP . '/app/code';
    $frontendHtmlFiles = getDirHtmlFiles($frontendDir);
    $appCodeDirFiles = getDirHtmlFiles($appCodeDir);
    $mergedHtmlFilePaths = array_merge($frontendHtmlFiles, $appCodeDirFiles);

    if ($runForFiles) {
        // Find and replace in emails (files)
        foreach ($mergedHtmlFilePaths as $k => $htmlFilePath) {
            $fileContents = file_get_contents($htmlFilePath);

            // Check if file is an email
            if (strstr($fileContents, '@subject') === false) {
                continue;
            }

            $mutatedFile = false;
            $hasCountedFileItem = false;

            // Find and replace in email content
            foreach ($variableReplaces as $oldValue => $newValue) {
                if (strstr($fileContents, $oldValue) !== false) {
                    if (!$hasCountedFileItem) {
                        $replaceFileItems++;
                        $hasCountedFileItem = true;
                    }

                    if ($dryRun) {
                        $replacedArray[$htmlFilePath][] = 'DRY RUN: Would replace file content: "' . $oldValue . '" -> "' . $newValue . '"';
                    } else {
                        $replacedArray[$htmlFilePath][] = 'Replaced file content: "' . $oldValue . '" -> "' . $newValue . '"';
                        $fileContents = str_replace($oldValue, $newValue, $fileContents);

                        $mutatedFile = true;
                    }
                }
            }

            // Overwrite file with new content
            if (!$dryRun && $mutatedFile) {
                file_put_contents($htmlFilePath, $fileContents);
            }
        }
    }

    if ($runForDatabase) {
        if (class_exists('Magetrend\Email\Model\Variable')) {
            /** @var Magetrend\Email\Model\Variable $mtModel */
            $mtModel = $objectManager->get(\Magetrend\Email\Model\Variable::class);
            $mtEmailCollection = $mtModel->getCollection();

            foreach ($mtEmailCollection as $mtEmailItem) {
                $mtEmailItemVarValue = $mtEmailItem->getVarValue();
                $mutatedMtItem = false;
                $dbTemplateId = "MT ITEM ID: " . $mtEmailItem->getEntityId();

                foreach ($variableReplaces as $oldValue => $newValue) {
                    if (strstr($mtEmailItemVarValue, $oldValue) !== false) {
                        $mtEmailItemVarValue = str_replace($oldValue, $newValue, $mtEmailItemVarValue);
                        $mtEmailItem->setVarValue($mtEmailItemVarValue);
                        $mtEmailItem->save();

                        $replacedArray[$dbTemplateId][] = 'Replaced text: "' . $oldValue . '" -> "' . $newValue . '"';
                    }
                }
            }
        }

        foreach ($templateCollection as $template) {
            /** @var \Magento\Email\Model\Template $template */
            $templateTitle = $template->getTemplateSubject();
            $templateText = $template->getTemplateText();

            $mutatedDbItem = false;
            $hasCountedDbItem = false;
            $dbTemplateId = "Database ID: " . $template->getId() . " (" . $template->getTemplateCode() . ")";

            // Find and replace in title
            foreach ($variableReplaces as $oldValue => $newValue) {
                if (strstr($templateTitle, $oldValue) !== false) {
                    if (!$hasCountedDbItem) {
                        $replaceDbItems++;
                        $hasCountedDbItem = true;
                    }

                    if ($dryRun) {
                        $replacedArray[$dbTemplateId][] = 'DRY RUN: Would replace title: "' . $oldValue . '" -> "' . $newValue . '"';
                    } else {
                        $replacedArray[$dbTemplateId][] = 'Replaced title/subject: "' . $oldValue . '" -> "' . $newValue . '"';
                        $templateTitle = str_replace($oldValue, $newValue, $templateTitle);

                        $mutatedDbItem = true;
                        $template->setTemplateSubject($templateTitle);
                    }
                }
            }

            // Find and replace in content
            foreach ($variableReplaces as $oldValue => $newValue) {
                if (strstr($templateText, $oldValue) !== false) {
                    if (!$hasCountedDbItem) {
                        $replaceDbItems++;
                        $hasCountedDbItem = true;
                    }

                    if ($dryRun) {
                        $replacedArray[$dbTemplateId][] = 'DRY RUN: Would replace text: "' . $oldValue . '" -> "' . $newValue . '"';
                    } else {
                        $replacedArray[$dbTemplateId][] = 'Replaced text: "' . $oldValue . '" -> "' . $newValue . '"';
                        $templateText = str_replace($oldValue, $newValue, $templateText);

                        $mutatedDbItem = true;
                        $template->setTemplateText($templateText);
                    }
                }
            }

            if ($mutatedDbItem) {
                $template->save();
            }
        }
    }

    if ($runForDatabase) {

    }

    $replacedFileCount = $replaceFileItems + $replaceDbItems;
    if ($replacedFileCount > 0) {
        print_r($replacedArray);
        echo "\n";
        echo "==================================================================";
        echo "\n";

        if ($dryRun) {
            echo "DRY RUN: $replacedFileCount items would have been edited. Files: $replaceFileItems, DB items: $replaceDbItems\n";
        } else {
            echo "$replacedFileCount items have been edited. Files: $replaceFileItems, DB items: $replaceDbItems \n";
        }
    } else {
        echo "Nothing has been replaced. Note: you can use '--files' and/or '--database' as variables to the file as a command.\n";
    }
} catch (Exception $exception) {
    $logger->critical($exception);
}

$time_end = microtime(true);

//dividing with 60 will give the execution time in minutes other wise seconds
$execution_time = ($time_end - $time_start) / 60;

//execution time of the script
echo 'Total Execution Time: ' . $execution_time . ' Mins';
echo "\n";
echo "==================================================================";

function getDirHtmlFiles($dir, &$results = [])
{
    $files = scandir($dir);

    foreach ($files as $key => $value) {
        $path = realpath($dir . DIRECTORY_SEPARATOR . $value);
        if (!is_dir($path)) {
            if (strstr($path, '.html') !== false) {
                $results[] = $path;
            }
        } elseif ($value != "." && $value != "..") {
            getDirHtmlFiles($path, $results);
        }
    }

    return $results;
}

Invoices don't seem to support the created_at_formatted variable, so that should probably be:

        'invoice.getCreatedAt()' => 'invoice.created_at|raw',
        'invoice.getCreatedAtFormatted()' => 'created_at|raw',
        'invoice.getCreatedAtFormatted(1)' => 'created_at|raw',
        'invoice.getCreatedAtFormatted(2)' => 'created_at|raw',
        'invoice.getCreatedAtFormatted(3)' => 'created_at|raw',

If we want formatting maybe we should make a filter for that. Currently it gives you date & time, we could make a filter to strip the time.