Altering the Present and the Past

Time travel is still pretty abstract at the present time and only possible in the realm of theoretical physics.  While the idea of traveling back in time and seeing the world as it was or traveling into the future (if possible, see my last post) to see what things will be like, there are some serioous moral and ethical issues to look at.  This technology could quite possibly be more dangerous than any nuclear weapon we could create.  Think about it, what would stop a rogue nation from going back to the mid-1700's to stop our founding fathers and preventing the formation of the United States?  Nobody would know because the course of history would be changed and we would all know it instead of what we know now.

What about the idea of going back and changing things only in one's life?  Surely there can't be any harm in that.  There can be though, especially if it involves life-changing decisions that would ultimately affect key moments in life such as marriage or children.  If one chose to pursue a college education rather than taking a factory job right out of high school, then he/she may not have met that special someone.  I would love to determine at least one possible path of events that would result from altering the past.  Even if it was meant as a noble deed, it could seriously alter history.

Of course, the idea of going back in time and altering key events does come with the question of what would happen to the memory of the one who does it.  If I went back and prevented the assassination of JFK, would my memories change?  I know I'm probably thinking through this a bit much, but it's one of my favorite topics.  What are your thoughts?