Every organism that exists or ever existed came to be through the interaction of its genome and its environment, which is essentially a huge complex of chemical reactions. So if you can edit genomes and control an organism's local environment (both possible), then you can produce at least anything that has existed and unfathomably many things that don't. That doesn't mean literally anything imaginable, but it does mean many, many things.

However, the ability to grow organisms with arbitrary characteristics requires biochemical knowledge far beyond what we have now. The technical limitations we currently face are nothing compared to the knowledge gap.

Moreover, the hypothetical scenario I'm talking about here involves creating an artificial genome in an artificial cell and then growing a macroscopic organism with an arbitrary body plan from it. That's theoretically possible, for sure, because it happens literally all the time in natural contexts.

But what you're describing with this hypothetical full-body genetic-level sex change of an adult human doesn't really make sense from a technical perspective. I mean, sure, it's theoretically possible to completely deconstruct a human body to the molecular level and then construct a new one, but that has nothing to do with genetic engineering. Remember that we're not talking about growing an organism from a single germline cell in this case - we're talking about restructuring every single somatic cell in a fully-developed organism. The composition and structure of tissues and organs are not determined by their cells' current genetic makeup (even if we include epigenetics); rather, they are the result of biochemical changes across their entire genetic history. Simply swapping out every single cell's DNA in an organism (even if we could do that) would not cause the organism to suddenly transform into the organism it would have been if it had had that genome from the start.

Here's an analogy: If you change the blueprints of a house before the house is built, then you change the house. But if you change the blueprints after construction, the house doesn't change. All you would do is cause problems for anyone who wanted to repair or remodel the house, because the plans wouldn't match the actual house. Can you tear down the house and rebuild it a different way? Sure. But that's a fundamentally different process.

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> Is it “it may be possible but the tech isn’t there” or “fundamentally, it is not possible”?

For a god it may be possible, for us, not so much. For comparison it's like modifying a program in a language we don't understand. This program has grown randomly over aeons and the strangest things relate to each other. You're basically stuck with randomly changing things and seeing what happens. But since people mature so slowly that's very slow progress.

During the maturation process of a foetus, there's lots of little tripwires to abort if something weird is going on.

That said, if you find some baby born with gill like structures or four arms, if you sequence them you might get a head start. But the slow testing phase will be a problem.

Evil science fiction villain mode: unless of course we figure out how to grow foetuses into babies without a woman being involved, you could build a factory to test 100,000 variations all at once. That would speed it up a bit.