You couldn't changed history. But you could get it right to start with. Do something differently the FIRST time around. This whole business with seeking Slytherin's secrets... seemed an awful lot like the sort of thing where, years later, you would look back and say, 'And THAT was where it all started to go wrong.' And he would wish desperately for the ability to fall back through time and make a different choice. Wish granted. Now what?