In
The War of the Worlds, H G Wells' narrator speculated that the squid-like Martians were actually a distant evolutionary descendent of a humanoid species. He suggested that the growth of technology and the ability of technology to replace biological functions with far more efficient synthetic augmentation would trigger an evolutionary drift towards optimisation of the form to only those aspects of the biology that were critically needed. This was, in the Martians' case, the brain, the senses and the hands (which evolved into the clustered tentacles).
The Martians, Wells put it "... were heads, just heads".
What is the relevance to this thread? Wells went on to speculate about a form of life that would 'put on bodies' according to need, these bodies being distant conceptual descendants of something as simple as the bicycle or walking stick. This could be applied to the idea of this topic. Imagine a human reduced to its most indivisible component - the brain and possibly the spinal cord, that could be transferred between various specialised chassis depending on need and the being's wishes.
This sort of extreme transhumanism is not unheard of in sci-fi as others have pointed out. One example of this is Anne McCaffery's brainships where you have a class of starship whose CPUs are volunteer quadriplegics whose brains are disconnected from their largely-destroyed bodies and connected into a 'shell', an interface unit that is then plugged into any number of chassis ranging from small courier-class spacecraft to
Citadel-sized space colonies. Interestingly, one of the 'pod people', Hyptia Cade, had an android body created that she could control using her ship's drone control subsystem. This enabled her to experience life outside of a hermetically sealed life support container for the first time in over a decade and even start a relationship with her pilot. Of course, the starship of which she was the organic CPU was another body that she used when required, its tractor beams, sensors, radio and engines replacing her hands, eyes, ears and legs.