I'm just backing up on my theory on whatever happened, happened! No matter what they do when they're in the past, it causes what happens in the future to happen.
Example- When Sayid shoots Mini-Ben, he tried to change the future to prevent Ben to join the Others/Hostiles. Instead, he caused it. No matter what you do it HAS to happen.
Another example-In another way, When Desmond was saving Charlie from death because of his flashes. People would say that he's altering the future by preventing his death, but really, his flashes and him saving Charlie because of the flashes, are part of time itself. Similar to the time travel on the island, them going BACK in time, is the timeline!
One more example- When Frank and Sun see the picture of the Dharma initiative with Hurley, Jack, and Kate in it. This proves that when they went back in time, it's part of the timeline. If they hadn't gone back in time, there wouldn't be a picture. But that wouldn't be different....because them not joining Dharma, would STILL BE THE TIMELINE!
Whatever happened, happened.