I really like the (relative, of course) plausibility of the technology in your stories. Too many "Technological" TFs are "Oh, we stuck some genetic material in you and now it takes over". Which isn't inherently bad, but it's nice to get some relatively plausible tech in TF stories.

Yeah, most people don't exactly realize that it would have to affect every single cell in your body for the DNA method to work, which would require more genetic material than could be fit in a syringe. Of course, there's always viruses, but those can be a bit unpredictable.

A virus is simply a segment of genetic code and a method for getting it into a cell. My understanding of genetics is that most genetic modification treatments today piggyback on highly modified viruses to get DNA into the target cells. In fact, some vaccines are actually viruses that have been modified to appear to be the virus we really want to protect against in the first place.
And, I believe, you could probably find a syringe large enough to fit all the DNA in your body. DNA doesn't take up THAT MUCH room. There's always novelty syringes, too.

This particular story does involve modifying the person's DNA as well as being in the cocoon, maybe that wasn't clear enough.

Modern genetic modification treatments only work on organisms on the level of individual cells; genetically engineered animals are the result of genetically altered eggs placed in regular animals. The problem with doing the same thing on the order of a fully grown individual of trillions of cells is double. Firstly, and more simply, it would take time, and having half your cells be from a different creature would almost definitely be fatal.

The second, and more fundamental, problem with DNA modification in fully grown individuals is the role DNA plays. While an individual is developing in the womb, DNA plays a critical role in the "design" (for lack of a better word) of said individual, by determining how cell multiplication plays out. However, once an individual is fully developed, DNA loses that role for the simple reason that the individual is already fully developed; DNA's other roles in the body (like protein production) become the primary ones.

Even if you simultaneously replaced the DNA in every cell in a human's body with engineered DNA, the human wouldn't magically undergo structural modification; instead, with DNA now unsuited for the body it's in, it would be fatal (along the lines of radiation poisoning, where DNA is damaged and no longer functions properly). In order to effectively transform an individual, you would need to both alter the DNA and, separately, alter the body structure, while somehow keeping the human alive in between. This is the role a cocoon could theoretically play.

This still isn't entirely realistic - there are far more obstacles in the way of a real transformation procedure - but it's one problem less, hence the "relative plausibility" referenced in my above post. (My personal favorite "plausible" TF method is growing a new body from scratch and transplanting the brain into it, which has the added benefit of getting around the philosophical problem of changing the brain.) Of course, a realistic TF method isn't essential, even to a sci-fi story, but it's nice to have a slightly more realistic one.

EDIT: I didn't even notice I wrote this whole spiel in response to the author. Huh.

I always like how your stories have rehabilitation in them in a way that just feels very cool to imagine experiencing. I was wondering for the whole story what the tone of voice of a naga would sound like, and how he would make sounds involving lips.