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 I think what you are asking is the basis for some of the best science fiction and fantasy novels and movies.  It always starts out in small increments, repairing debilitating disabilities to purging undesirable traits and limitations.  At some point something is lost.  Do we lose our humanity as we switch out our body for machine or is it in the mind?  And is our mind's desire to constantly improve itself enough to remain human when it has tampered with its physical presence in the brain?  I guess where we draw the line depends on what we think humanity is, and even more so, what we value about it.