startup.php

<?php  

\Pimcore::getEventManager()->attach("object.preUpdate", function (\Zend_EventManager_Event $event) {
    $newObject = $event->getTarget();
    \Pimcore::collectGarbage();
    $oldObject = \Pimcore\Model\Object::getById($newObject);
    
    if($newObject->getKey() != $oldObject->getKey()) {
        // ... 
    }
});