It works for me
1. import EntityManager
2. Now you can use this anywhere into the class.
use Doctrine\ORM\EntityManager;
$preData = $this->em->getUnitOfWork()->getOriginalEntityData($entity);
// $preData['active'] for old data and $entity->getActive() for new data
if($preData['active'] != $entity->getActive()){
echo 'Send email';
}