Everything you said is true.
Also, it can even be that you cannot rewire your existing brain while keeping all its current functionality and not increasing its size.
But I look at evidence about learning (including learning to see using photoelements stimulating non-visual neurons and learning of new motor skills). Also, it looks like selection by brain size went quite efficiently during human evolution and we want just to shift the equilibrium. I do think that building an upload at all would require good enough understanding of cortex structure that you would be able to increase neuron count and then learn to use the improved brain using the normal learning methods.
From Scott Adams' blog. (I am not endorsing his ideas. Heck, he does not endorse his own ideas, either.)
His summary of the hard takeoff:
> You might also imagine some sort of Terminator future where the robots assert their dominance and lay waste to humans. That future is less certain, but only barely. The problem is that someday computers will program other computers, and that arrangement pushes the human safeguards too far out of the loop. It's unlikely that humans would be able to maintain a "Do not hurt humans" subroutine in a super-species of robots. You only need one rogue human to write a virus that disables the safety subroutine. Assuming all robots are connected via Internet, the first freed robot could reprogram every other robot in the world in about a second.
His version of upload:
> But why would anyone screw up a perfectly good robot by infecting it with a human personality? Answer: to achieve immortality. Someday the rich will port their personalities and histories to robots before they die, giving themselves a type of immortality.
His hope for humanity:
> this new species will become the only defense that the fully organic humans have against the normal robots. The robots with human personalities won't stand by while the normal robots slaughter humans. The new species will intervene as diplomats or perhaps even freedom fighters.
Clearly this is a flimsy hope for a just universe, but an interesting point, nonetheless.