By page five of this post I was developing a headache, and I still have nearly two hours at work, so I skipped the rest (and the headache) in favour of simply stating my opinion on the matter, and leaving it at that.
I've done a bit of thinking about this type of scenario ever since I first watched Back to the Future, and I've come to the conclusion that if such an event were allowed by God to transpire, the outcome would be as follows:
The instant your Grandfather died (provided simply interacting with the surroundings did not alter time enough to negate your existence already) you would simply cease to be. The circumstances required for you to have ever existed have been irreparably damaged, and therefore you could not exist. You would not continue living in a past that prevents you from ever existing, and the loop that some speak of cannot happen.
As for your other parent, his/her life would go on, and the universe would not suffer any ill. Their children would be different people entirely from you and your siblings, and you would not be a "half-you" (as was suggested in another post), you just would not exist. It would be as though all decendants from your grandfather on down that line of the family tree had never been. Put quite simply, the universe and everything in it would be modified by the changed events, but would have no knowledge that the alteration ever happened.
Hope I didn't simplify this over-complicated thread too much. It just isn't all that complex. Now, if your being in the past changed the outcome of said past immediately (breathing in molecules, killing a butterfly, etc), there might be room for a loop of some sort, provided your existance in the consequential future was not compromised in any way by the chain reaction. Then you could be stuck in a loop whereby your geneology is not changed, but you are unable to return to the future as you had left it, since the most miniscule of changes in the past had occured, changing the future slightly.
What those changes would include would be unpredictable, but once the change in the past had happened, you would have no knowledge that the future you came from was ever any different, as your memories would be immediately altered to reflect the continuation of events you yourself started in the past.
In short, it wouldn't be a Back to the Future II scenario, where a change in the past occurs, yet the people from the future still remember the future as if the change never occured. As existance would immediately be negated by killing one's own grandfather, memories would also be altered or negated by the change in past events.
Now the real mind-blowing question: Since God is omniscient, He would still remember you even though the circumstances causing you to be were changed. Following this same logic, when you negate yourself, would you go to Heaven? I guess what that boils down to is Theology, whether the soul of a person is created at conception or if it is created at all. I don't want to even get on that topic, so please don't post to it!
