If you replaced every single cell of your body one-by-one with a nanomachine, at what point are you not human?

That is a very good question !. You first have to look at what makes human. Most would usually say our consciousness,free will and ability to think intelligently. You could probably replace all of the body cells apart from the parts if the brain that control of emotion and consciousness. Every other part of the body is just a "biological machine" and every action from cell and protein action to body movements is just a series of "mechanical actions". Proteins are responsible for many bodily functions which is done by mechanically changing shape "conformation" and interaction physically with other proteins or tissue. Communication is primarily done by electrical nerve impulse and proteins known as "messenger proteins that allow transmembrane communication . So replacing all but the sense of emotion and free will I think could still be called as humans. As an afterthought, I think also certain DNA strands responsible for the above. There is also the question of reproduction, although mostly mechanical, there would still be the DNA information that is required for development of personality, emotion etc so this may also have to remain biological. All in all this a great question and given me a lot to think about.

I think that point would be reached when there is no longer any common ground with humans. If you can't feel, think, taste, hear, speak, etc. from a common biological, psychological, or physical perspective what basis of commonality could you base the "humanness" on? Our being human is defined by our shared evolutionary history and our adapted methods of communication to others who are similar enough to be able to match our patterns to patterns within themselves. It will be exciting to find out definitively where that point is. Terrifies and intrigues me lolz.

I would say when your brain becomes a machine you are not human, because then a robot basically is controlling you or your robot self is controlling your human self.

Since you asked in philosophy, it can only be answered by saying this:

When it happens, it will up to medical ethics to determine this, but such things always end up bringing in legislation to make it law.

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