Ah man, Time Travel.
I'm more of the camp that even if you did go back in time to change something, the odds are that you wouldn't change a thing - something would happen to prevent you from doing what you intended to go out to do.
Like, for instance, let's say you really are hell bent on going back in time to the JFK assasination dressed up in Chicken Suits. This is what you plan to do, and there is no way you are going to change your mind.
Even before you set out (in the present time) to do it - it's already happened. You were already there at the JFK assasination anyway. Whatever the circumstances were, you didn't manage to divert the camera mans attention away onto yourselves.
If you had a time machine that would allow you to go to a certain place at a certain time, and you programmed it to take you back to that time right in the middle of the road - something would have gone wrong with the device and it would have transported you somewhere else entirely from your intended place.
This is, unless, you are bringing alternate dimensions and universes into the matter, at which case you'd have left our world and gone into one where the JFK assasination was thwarted by 25 men dressed as chickens
