Posthumans: Ship of Theseus Paradox

The Ship of Theseus is a paradox also known as Theseus' paradox. It raises the question of whether an object, which has had all its component parts replaced, remains fundamentally the same.^wikipedia. The Ship of Theseus is a good representation of a future technology in which you can choose to become replaced by manufactured parts, even the parts of your brain.

At what point does a human become posthuman? Steven Pinker, a cognitive neuroscientist and author of How the Mind Works, poses the following hypothetical, which is an example of the Ship of Theseus paradox:

Surgeons replace one of your neurons with a microchip that duplicates its input-output functions. You feel and behave exactly as before. Then they replace a second one, and a third one, and so on, until more and more of your brain becomes silicon. Since each microchip does exactly what the neuron did, your behavior and memory never change. Do you even notice the difference? Does it feel like dying? Is some other conscious entity moving in with you?
-How The Mind Works, Steven Pinker, p. 146

We already replace our cells in our bodies, all the cells, in a surprisingly fast rate, and have little problem maintaining a sense of coherent continuance. So the Ship of Theseus paradox already occurs biologically.

What would be a new phenomenon is the integration of manufactured parts into the fabric you call you.

Made of eternal parts, or at least able to replace parts eternally.

Assuming the eternally functioning brain and body do eventually happen, the implementation and distribution amongst humans could create utopia or dystopia. What if some humans war against the Eternalists? What if the technology becomes widely adopted, but there are stealthy serial killers amongst the Eternalists? The sci-fi novel possibilities are endless.