Here's one. You CAN'T change the future as we see with Desmond's attempts to save Charlie. But you CAN change the future also based on what we see with Desmond's attempts to save Charlie.
Think about it, had Desmond not failed to build a lightning rod, or save Claire, Charlie would've died earlier than he did. If this happened, he never would've gone down to the Looking Glass and deactivated the jammer. He never would've been able to warn them that it wasn't Penny's boat. The losties wouldn't've had a clue when a copter full of mercenaries arrived to get Ben and kill the rest.
So, can you change the future? No, Charlie couldn't be saved.
So, can you change the future? Yes, by postponing events long enough to get a different effect.