Altered Carbon used alien magic, the way this works in the real world will be far worse: Brain transplants. First, many poor people will need to be used as guinea pigs (a la the Sun King's anal fistula). Then once it works.. well some strapping young man (or woman!) will have to "volunteer" their body to host Elon's brain.
They don't need a volunteer. They can clone themselves when they are, say, 30 year old and they will have a 100% compatible, 20 year old donor who has spent their life in suspended animation when the original is 50.
The practice of illegal clone brain transplants figures in some of the Vorkosigan series books: The clone-children of various customers are raised in cohorts, and taught little while enduring years of strictly controlled diets, cosmetic surgeries, and exercise regimes.
Then, one day, the are told their important and distant "parents" are finally arriving to bring them away to their new life...
Anyway, the point is that any aging wealthy pedicidal murderers are also gonna insist the body is perfect before they move in. The easiest way to do that without conjuring more new technology is the force the future-victim to do it.
Thanks for bringing that up, it's probably time got for me to reread the Vorkosigan Saga.
And you also reminded me of the flawed but moving film "Never Let Me Go" from 2010 about a more present version of this. Oh, and there's also Michael Bay's "The Island".
No that won't do. You need someone else to prepare the body: you know... rigorous workouts for strength and physique. Ideally the person is an excellent cage fighter and has the reflexes of a top-tier video gamer.
All options are too far away to predict which will come first, or with what side effects.
(In practice, almost everything over 5 years away, even when already in early human trials, has this property; the only reason the Covid vaccines happened faster is that everyone was willing to throw unlimited resources at the problem and do simultaneous tests on all candidates, and in a pipeline, rather than cost-efficiently and slowly like everything else has been).
IIRC, there's no current way to scan even a single living synapse/synaptic cleft/dendrite combination to read out the corresponding connection strengths, let alone for the whole brain, so we can't yet scan a brain — but if we could do that, writing it back to a fresh blank one currently seems(!) like the easy part, as neurons change shape and grow in response to electrical gradients.
One non-measuring idea is to gradually replace portions of the brain with artificial blanks, relying on some sort of holistic (or holographic) redundancy where the "damage" is repaired by neighbors.
This, er, Brain of Theseus would retain operational patterns even if the individual cells have been replaced.
A variation on that would be too do it stochastically, constantly substituting a miniscule percentage of cells evenly across the entire brain.
