I don't have time to check all the other theories so if someone has already posted a theory like this than I apologize. I think that they will be able to change things, but only for the future. They cannot change anything for the time that we the viewer have already seen or have already been told the history of (Dharma, for example). The incident will play out the way it always had, whether that be with Jack's interference or not (since with this theory, Jack always interfered). They will somehow get back into the same time as Locke, Ben, and Sun, but from that point on, the slate will be cleaned. Let's just imagine that people like Widmore, Mrs Hawkings, Jacob, and possible Ben have always known what is suppose to happen and have watched it play out time and time again. But for whatever reason, whether it be Desmond or something else entirely, the future has been changed. Just like Faraday and Chang have ! said before, you can't change the past, but maybe you can change the future. Remember the Mrs Hawking told Penny that for the first time in a long time, she doesn't know what's going to happen. Now those people that have known how things are going to turn out are just as clueless as the rest of us. And this way, you don't mess with the integrity of the first 4 seasons because a lot of people feel that if you change things that we've already seen, then it makes those first 4 seasons null and void. We the viewer are left with the satisfaction of knowing that while things that we've already seen will NOT change, the future is not pre-determined. They can change whatever end-game war that has been alluded to and possibly live happily ever after (yeah right!). I hope I've explained this well enough since I'm at the tail-end of a long day:)
