(My first blog post, sorry if it's somehow incorrect)
Now correct me if I'm wrong, but I think this proves that Jack and crew caused the Incident, and that whatever happened, indeed happened.
On the day of the incident, Pierre Chang was supposed to be working on the Arrow Orientation filming (plus visiting the Orchid station). If Daniel hadn't warned him of the Incident, and if Hurley hadn't admitted that they were all from the future, Chang would have no reason to be at the Swan Station, which is where he injured his arm during the Incident. But as we know from Swan Orientation video from season 2, Chang has a prosthetic arm, from the Incident (confirmed again by the Dharma Booth Video, even though it's not entirely canon). If the Losties hadn't interfered, Chang wouldn't have lost his arm, which happened in the Losties past. Therefore, whatever happened, happened.
Additionally, if Miles wasn't there, Chang would probably have died from his injury, being stuck between two metal contraptions being sucked into the electromagnetic pocket. If Miles wasn't there in the past to save him, the 2004 Losties from Season 2 wouldn't see Pierre Chang with a prosthetic arm in the Orientation videos.
Are there contradictions to this theory, or is it indeed that whatever happened, happened?