Daniel Faraday said that u cant change the past but the future. So if u went in the past of yours, then nothing would happen in the future. But if u got into a past u were never in, u would change the future not the past. Like when Locke and the others were in the 50 s, Locke when met Richard and Widmore in their camp. And then when Locke teleported off the island, Widmore met Locke in Tunisia and said he met him when he was 17. But that would mean Danielle would remember Jin from the past.