First paragraph summarizes it clearly. Death of petitioner after petition has been approved may qualify for humanitarian parole. Specifically, In your dad's situation he just can't switch primary sponsor; that's not how it works. He needs USCIS permission to do so. You see, generally, family petitions are for family unification/reunification inside the US. So if the petitioner is dead there's no longer a need for that petition because reunification is not possible. There are exceptions to this but again your dad's situation is not one of those exceptions that's why you'll need a humanitarian parole reinstatement at USCIS's discretion. Good luck.