There is a lot of misinformation out there, and we all have our favorite pet peeves.

And you are exactly right. You could take a cell sample from a grumpy stubby grizzled fat old man, culture it, and bio print it into a delicate fair maiden. Or an elf. Or a kitty girl. Or anything biologically viable. I'm ignoring the part about creating stem cells and causing them to differentiate into the appropriate tissues, but you get the idea.

In an earlier (and unpublished) SF story, I used a virus to add another pair of chromosomes that were carefully designed from the original twenty-three pairs. It caused the cells to essentially recreate the body from the original 23 the same way that a zygote turns into a baby. Lots of handwavium, but eh, it made for a good story.

As the good doctors Martin pointed out, the DNA that builds the body is quiescent once the body has been built.

But there are lots and lots of errors in science fiction. We even have one of the Big Three using regular rockets to thrust until light speed is reached. I knew that that was wrong way back in junior high when I read it, but the story was still good. Even if that error annoyed me.

EDIT:
The whole bit of modifying an adult organism isn't fiction. Caterpillars do it all the time. That's why I chose to use that method with the above mentioned SF story. Come to think of it, I used it in two stories. Neither have been published.

Yes, I'm also dissatisfied with stories that seem to assume that if you replace a Y chromosome with an X chromosome, it will turn an adult (or even child) from male to female. To me, this shows a Fox-news level of understanding of biology.

For one thing, anatomical sex determination is a heck of a lot more complicated than X or Y chromosomes; intersex conditions such as CAIS are an example, but (according to Wikipedia, at least), there are even (very rarely) fertile XY females and XX males. Throw into the mix things like XXY and chimeras (people with a mix of XX and XY cells), and it gets even more complicated.

For another, the DNA (and epigenetics) don't contain a blueprint for the finished product. They contain programs for developmental steps, which, if all of them work out the way we assume they're supposed to (DNA doesn't have a "supposed to," it just is), result in a body that more or less conforms to our idea of a "female" or "male" body (and if any don't, you end up with a miscarriage or an intersex individual.) It's the difference between on the one hand having the latitude and longitude for Times Square and heading in the right direction until you get there, and on the other hand, following an extremely detailed set of instructions which, if followed exactly, will put you at Times Square, but if you mess up one step, you end up in the East River, and if you mess up a different one, you end up in a suburb of New Haven, Connecticut.

Converting a more or less grown male or female human body into what we consider a normal body of the "opposite" sex would require a completely different set of developmental steps. So you'd have to have DNA that is not at all like anything in any human being's body, encoding a developmental "program" that you'd have to invent from scratch. Moreover, such a program (if it didn't kill the patient) could turn a body into practically anything. It would only look like what we think of as a typical female (or male) body if you had a very detailed understanding of the body you were trying to create -- and you were very good at implementing it and getting around all the technical difficulties. It might well turn out to be easier to turn someone into a body that looked mostly like a typical female/male body but left out some of the harder things.

In the one story I wrote which involved such a transformation, I imagined that someone had indeed come up with such a "program" and created virus-like things that would take over a person's cells and do all the complicated stuff required to do the transformation. I presumed that this would be an outgrowth of research into ways to get the body to regrow organs and body parts lost due to disease or accident, because human beings don't have "programs" to regrow limbs, either, so you'd need to invent a developmental path for regrowth and implement it somehow. I also assumed that the creators of this "program" would focus on getting the details right that their patients cared about most and do whatever was easiest for the rest. In my story, I assumed that creating ovaries and a uterus was too hard, but having something that bled a little on a regular basis was not.

(One of my readers took issue with this, saying that you couldn't have monthly bleeding without uterus and ovaries, which baffled me. Since it was all just stuff I made up and was and is so far beyond our current capabilities that we have absolutely no idea what would be involved in any of it, why would my assumption be any more implausable than any other?)

Oddly enough, I find stories that use "magic" as an explanation for how a boy gets turned into an anatomical girl (or the other way around) easier to accept. "Magic" is by definition outside of anything we have in the real world, so it doesn't clash with my knowledge of how things are. Even if someday someone really does invent a treatment for turning a boy into a girl or vice versa, it will be so far from anything we know about that it might as well be magic.

And when you come down to it, why does it matter? I don't think anyone reads these stories to learn about real-world techniques for changing their body into a female body (or the other way around), do they? I think we read them so we can imagine being the gender we wish we were, and the details of how we get there aren't all that important. All we ask of the explanation is a bit of pausible hand-waving that helps set up the rest of the story.