Significantly changed? So you think that the past can be changed, but not in a way which changes the present? In other words, you believe that small changes are possible? I disagree. I see no reason to think that the past cannot be changed, except if it's just a tiny bit. Don't see the logic in that at all. Also, I believe Chaos theory applies here; a butterfly batting their wings can cause hurricanes on the other side of the earth(or time, in this case).

Anyway, I rather believe that distinction between past, present, and future are merely in our heads. But yeah, just read on.