Science fiction writers, as always, lead the way into the future. In Waldo, published in Astounding Science Fiction in 1942, Robert Heinlein describes a man afflicted with myasthenia gravis, a neuromuscular disease resulting in extreme physical weakness. Heinlein gives him his own private space station, nicknamed by others Wheelchair:

Waldo F. Jones seemed to be floating in thin air at the center of a spherical room. The appearance was caused by the fact that he was indeed floating in air. His house lay in a free orbit, with a period of just over twenty-four hours. No spin had been impressed on his home; the pseudo gravity of centrifugal force was the thing he wanted least. He had left Earth to get away from its gravitational field; he had not been down to the surface once in the seventeen years since his house was built and towed into her orbit; he never intended to do so for any purpose whatsoever.

(Waldo in his space station 'Wheelchair' (Heinlein))

Here, floating free in space in his own air-conditioned shell, he was almost free of the unbearable lifelong slavery to his impotent muscles. What little strength he had he could spend economically, in movement, rather than in fighting against the tearing, tiring weight of the Earth's thick field...

Another example is Commander Doyle in Arthur C. Clarke's Islands in the Sky (1952):

I was feeling a bit scared, and to make matters worse I’d drifted away from the floor and was floating helplessly in mid-air again. There was no way I could move anywhere unless I made myself ridiculous by trying to swim, which might or might not work. Then the commander gave a chuckle, and his face crinkled up into a vast grin.

“I think this may be quite amusing,” he said. While I was still wondering if I dared to ask why, he continued, after glancing at some charts on the wall behind him: “Afternoon classes have just stopped. I’ll take you to meet the boys.” Then he grabbed a long metal tube that must have been slung underneath the desk, and launched himself out of his chair with a single jerk of his huge arm.

He moved so quickly that it took me completely by surprise. A moment later I just managed to stifle a gasp of amazement. For as he moved clear of the desk, I saw that Commander Doyle had no legs...

It was a long time before I discovered what had happened to the commander. The scar he’d picked up in an ordinary motor crash when he was a young man, but the more serious accident was a different story, having occurred when he was on the first expedition to Mercury. He’d been quite an athlete, it seemed, so the loss of his legs must have been an even bigger blow to him than to most men. It was obvious why he had come to the station; it was the only place where he wouldn’t be a cripple. Indeed, thanks to his powerfully developed arms, he was probably the most agile man in the station. He had lived here for the last ten years and would never return to earth, where he would be helpless again. He wouldn’t even go over to any of the other space stations where they had gravity, and no one was ever tactless or foolish enough to suggest such a trip to him.

Update 25-Nov-2022: In John Varley's 1977 novel The Ophiuchi Hotline, one of the characters has modified herself in a radical and unique way to make it easier to live and work in space:

It was uncanny, beyond belief, how Javelin had threaded herself through the seemingly impassable maze.

Lilo looked at Javelin and saw a two-meter cylinder, swelling gently from the extremities to a fatter part in the middle, with a hand at each end. The cylinder was flexible at four points, which were her knee, hip, shoulder, and elbow. Growing from her “shoulder” at a slight angle from the rest of the cylinder was her head, with brown hair cut efficiently short. She wore a simple blue tube of cloth that left her arm and leg bare.

That was Javelin, with her arm held straight up. When she put her arm at her side, she looked like a jackknife.

What she had done was not a simple matter of getting rid of her right arm and left leg. Dispensing with two limbs—usually the legs—was common among spacers. But rib cage, right shoulder, and left hip had been redesigned with plastic structures replacing the bones. She had got rid of her left kidney, right lung, and a lot of intestine. Her elbow and knee had been reengineered with ball and socket joints.

She was limber as a snake. What was left of her could wriggle through a hole twenty centimeters in diameter...

They got the items moved into the scooter. It might have gone faster, but all three were fascinated by Javelin’s movements. She would grab a handle at the side of the lock with one hand, reach out with her leg and use the hand on that end to snatch a piece of furniture, pull, and bend like an eel as she guided it through the hatch.

I read that book when it came out, and this scene is one of the things that stuck in my memory, although I couldn't place the source. Thanks again to @sailor_peter, @NelC, N_J_Bennett and especially @CptButton who chimed in with a text quote.

Another example of deliberate body modification used by people who work and live in space can be found in Babel-17 by Samuel R. Delany; see the entry for antigravity globe arena, a kind of wrestling event that allows these individuals to showcase their abilities.

There was a quadruple-amputee astronaut named Frank in Fred Saberhagen’s Berserker Man (1979):

Most of the space in the commander's small cabin was occupied by Frank's acceleration couch and by his body. It was not easy to see just where the one ended and the other began. Photographs Eli had seen of Frank, made before that brush with the berserker nine years back had almost cost him his life, showed a trim-waisted young-looking young man, so intense that even his image seem to endow him with extra energy. Now what the berserker and the surgeons had left of that vital body was permanently cushioned in fluid and encased in armor.

The three cable connected units in which Frank lived struck Eli sometimes as a lazy costumers' concept of an insect body. There were head, thorax, and abdomen, but no face to turn to Eli as she entered. She knew though, but Frank would be watching her with a part of his instrument perceptions while he remained wired directly to the sensors of the ship, and adequately alert. One plastic and metal arm rose from the central box to acknowledge her presence with a small wave.

Thanks to @JimSharkey for contributing this reference. End update.