Short story: planet with wrong chirality
"Contagion", a 1950 novelette by Katherine Maclean, apparently also the answer to this old question. You can read it at Project Gutenberg or the Internet Archive, or listen to a reading at LibriVox or YouTube, or you can read a review by Doomsdayer.
The explorers landed on a planet where the chirality was opposite that of Earth’s.
Patrick Mead, one of the original colonists, explains to the new arrivals:
"Chemical differences in the basic protoplasm of Minos. Different amino linkages, left-handed instead of right-handed molecules in the carbohydrates, things like that. Nothing will be digestible here until you are adapted chemically by a little test-tube evolution. Till then you'd starve to death on a full stomach."
For some reason they couldn’t leave,
Plague. "What was the disease?" Hal Barton asked.
"Pretty gruesome, according to my father. They called it the 'melting sickness.' The doctors died too soon to find out what it was or what to do about it."
"You should have trained for more doctors or sent to civilization for some." A trace of impatience was in George Barton's voice.
Pat Mead explained patiently, "Our ship, with the power plant and all the books we needed, went off into the sky to avoid the contagion, and never came back. The crew must have died." Long years of hardship were indicated by that statement, a colony with electric power gone and machinery stilled, with key technicians dead and no way to replace them. June realized then the full meaning of the primitive sheath knife and bow.
so one of the scientists isolated people’s cells, basically put them into an aquarium and added samples of the alien plant life so that the cells could evolve to make use of the wrong chirality. He then injected them back into the original people.
"It's a story." Pat leaned back again. "Alexander P. Mead, the head of the Mead clan, was a plant geneticist, a very determined personality and no man to argue with. He didn't want us to go through the struggle of killing off all Minos plants and putting in our own, spoiling the face of the planet and upsetting the balance of its ecology. He decided that he would adapt our genes to this planet or kill us trying. He did it, all right."
"Did which?" asked June, suddenly feeling a sourceless prickle of fear.
"Adapted us to Minos. He took human cells—"
She listened intently, trying to find a reason for fear in the explanation. It would have taken many human generations to adapt to Minos by ordinary evolution, and that only at a heavy toll of death and hunger which evolution exacts. There was a shorter way: Human cells have the ability to return to their primeval condition of independence—hunting, eating, and reproducing alone.
Alexander P. Mead took human cells and made them into phagocytes. He put them through the hard savage school of evolution—a thousand generations of multiplication, hardship, and hunger, with the alien indigestible food always present, offering its reward of plenty to the cell that reluctantly learned to absorb it.
"Leucocytes can run through several thousand generations of evolution in six months," Pat Mead finished. "When they reached a point where they would absorb Minos food, he planted them back in the people he had taken them from."
I think when the back-up team came
Not a "back-up team", the new colonists who arrived three generations later were surprised to find the old colonists there.
all of the original explorers had the same body (copies of the one person whose cells made the transition correctly)
June stepped closer and stood looking up at him, considering her own description. She was tall and tanned, like him; had a few freckles, like him; and wavy red hair, like his. She ignored the brightly luminous blue eyes.
"In other words," she said, "everyone on the planet looks like you and me?"
Patrick Mead took another look at their four faces and began to grin. "Like me, I guess. But I hadn't thought of it before. I did not think that people could have different-colored hair or that noses could fit so many ways onto faces. . . ."