startup.php

<?php

\Pimcore::getEventManager()->attach("admin.controller.postInit", function ($event) {
    $controller = $event->getTarget();
    if($controller->getParam("controller") == "object" && $controller->getParam("action") == "update") {
        $object = \Pimcore\Model\Object::getById($controller->getParam("id"));

        // do with the object whatever you want
        // later in ObjectController::updateAction() the pimcore admin will get the same instance of this object
        // since ::getById() returns always the same object for an ID
    }
});