code: func (r *EtcdRegistry) UpdateEngineVersion(from, to int) error {
  	key := r.engineVersionPath()
  
  	strTo := strconv.Itoa(to)
  	strFrom := strconv.Itoa(from)
  
  	opts := &etcd.SetOptions{
  		PrevValue: strFrom,
  	}
  	_, err := r.kAPI.Set(context.Background(), key, strTo, opts)
  	if err == nil {
  		return nil
  	} else if !isEtcdError(err, etcd.ErrorCodeKeyNotFound) {
  		return err
  	}
  
  	opts = &etcd.SetOptions{
  		PrevExist: etcd.PrevNoExist,
  	}
  	_, err = r.kAPI.Set(context.Background(), key, strTo, opts)
  	return err
  }
docs: // UpdateEngineVersion implements the ClusterRegistry interface