We must remember a very wise old women named Eloise Hawkings. When Desmond turned the key his mind was sent back in time two a younger version of himself. He meets Eloise whilst trying to change the past. She explains to him that "the universe has a way of coarse correcting". This concept is proven when desmond tries to save Charlie. It was as if he was trying o change a past that had not happened yet.
SO WHAT DOES THIS MEAN!?!?!
It means that whatever happened, happened only it doesn't have to happen a specific way. The universe will treat the entire island as if it were Charlie. Evan if Jack changed something, it WILL change back.