I've been rewatching season 4 on DVD, and I was thinking about 'The Constant" which is my favorite episode of the series. Daniel actually tells Desmond that you can't change the future, and when they first meet in 1996 he mumbles something about a paradox. Desmond was also told when he first time travel experience that he couldn't change things, and that the universe would course correct.
From all the evidence in the show, in "The Constant" Desmond did change the future. Desmond was able to talk to Penny because her got her new phone number back in '96. If he had not done that, he would have died. He wouldn't have a constant and would have ended up like George.
That's just one small way that he changed the future when he called Penny. Desmond is responsible for the actual rescue of the Oceanic 6, because Penny comes out to get Des after talking to him on the phone.
Anyway, Desmond has time traveled twice now, and has changed the future. How will things change this season since it appears that the island moved through time and space?