public void upgrade() {
    if (myInstanceLow == null) return; // already upgraded
    myInstanceHigh = myFactoryHigh.createFrom(myInstanceLow);
    assert myInstanceHigh.size() == 0 || myInstanceHigh.size() == myInstanceLow.size();
    if (myInstanceHigh.size() == 0 && myInstanceLow.size() > 0) {
      for (IntIdsIterator it = myInstanceLow.intIterator(); it.hasNext(); ) {
        myInstanceHigh.add(it.next());
      }
    }
    myInstanceLow = null;
  }
public void upgrade()