Personally I believe they haven't changed anything, but here's a thought.
If they flash back to being on the plane, I believe one of two things could easily occur.
The first being that Dharma is successful in changing one of the numbers, continuing their experiments with even more government funding now, may still be wiped out by Ben, and any hatch or other experiment could cause the crash. This crash however kills many more people, as dead is dead. This would include Shannon, Boone, Locke, Charlie, they'd all die again, except in the crash.
The second being that they land in LA airport, only somewhat knowing the people around them, but scared to make a scene by greeting them after the island. Kate goes to jail, Jack goes to his father's funeral, Charlie is arrested, Locke goes on to kill himself, Sawyer does whatever as usual, Hurley visits his mother, -Claire gives up Aaron. Sun and Jin on the run. And Rose will die. Then pesky little Jacob(or his nemesis) shows up to cause a little ruckus(maybe before they leave the airport).
Or they could always end up in the 1950's. I here it's nice there.