oca-ocb-sale/odoo-bringout-oca-ocb-event_sale/event_sale/data/mail_templates.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>

<template id="event_registration_change_exception" name="Message: Alert on event registration data change">
    <div>
        <p>
            <span>Registration modification for attendee:</span>
            <a href="#" data-oe-model="event.registration" t-att-data-oe-id="registration.id"><t t-out="registration.name"/></a>.
            <span>Manual actions may be needed.</span>
        </p>
        <div class="mt16">
            <p>Exception:</p>
            <ul>
                <li>
                    <a href="#" data-oe-model="event.registration" t-att-data-oe-id="registration.id"><t t-out="registration.name"/></a>:
                    <span><t t-out="record_type"/> changed from <strong><t t-out="old_name"/></strong> to <strong><t t-out="new_name"/></strong></span>
                </li>
            </ul>
        </div>
    </div>
</template>

</odoo>