how do you are not introducing a new force into the system by just observing it! the event and likely chance of change is relative to the size of the event.. for example, you want to go back in time and watch a star being born. the forces necessary to create are star are very large, and so as an observer present in the system, you will not likely change anything.
BUT, what if you went back to prove the beginnings of evolution.. you have determined the point at which the first proteins combined. you as an observer may be a large enough force for that event, that you could change the outcome.. a different protein, no protein, or it could still stay the same.
now, as for going back to an event where people or something besides just forces are involved, then u encounter new issues. i want to see who was really on the grassy knol... well, by me staring at the spot, the shooter worried and either doesn't show up, or goes somewhere else. by merely observing, i have changed the outcome.
if you believe in the theory/possiblity of infinite universes (parallel ones), then each observer/observation taken only travels a different path, since all possibilites are already occuring.
if there is a way to "view" the event without adding a new force to the system, then the outcome should be the same... though i don't think that is possible.. there would still be some small force added just by the viewing alone.