"It can't change if it hasn't already happened, can it?" Luna pointed out, wisely. "But some outcomes can be avoided, if you know what might happen. It's really very confusing to me, but Cesc is never distressed by it. Sometimes, it looks like one thing is going to happen, and then a decision is made that changes it. But the major endings tend to remain the same. I mean, in Cesc's original vision, I was killed opening the portal, but now I'm not. But the portal still gets built and opened and Harry sends Riddle through it later. That's the main thing, and that sort of stuff doesn't seem to change. It's like Point A and Point Z remain constant, but that the other letters can get muddled up an infinite number of times. It's the journey between the start and finish that changes, but the important stuff doesn't."