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?
I think writing should be about change.