How to automatically send a transactional email on customer group change?

Hi you can do this by magento event Observer and find a trigger an observer on customer save after and fire send your custom transactional email automatically.

You can try this Two Event

1. customer_save_before

2. customer_save_after

Find details on event and observer here

Model Approach

Step1: trigger an event on basic of customer_save_after

<global> <events> <customer_save_after> <observers> <stockalert> <type>singleton</type> <class>magento37890/observer</class> <method>checkDisplay</method> </stockalert> </observers> </customer_save_after> </events> </global> 

Step2: Magento Observer Code is

<?php class Stackexchange_Magento37890_Model_Observer { public function checkDisplay(Varien_Event_Observer $observer){ $customer=$observer->getEvent()->getCustomer(); Mage::log('My log entry'.$customer->getId(), null, 'Magento37890.log'); /* if customer is old customer */ if($customer->getId()){ if($customer->getOrigData('group_id')!=$customer->getData('group_id')){ /* here you write your code to send whenever you have change the group */ } }else{ /* New Customer */ } } }