There's a couple issues here: this would be fairly difficult, as you'd need to change the DNA in probably several billions of cells before any phenotypic change

There's a couple issues here: this would be fairly difficult, as you'd need to change the DNA in probably several billions of cells before any phenotypic change could be effected. Perhaps one mechanism may be to engineer a retrovirus that would rewrite portions of DNA. This could obviously have far more important impacts as well: such an RV could reprogram cancerous cells to die off like proper cells, or disable their angiogenic capability; and of course it could be programmed to rewrite genes implicated in diseases like sickle-cell anemia or multiple sclerosis. Another issue is that while genetics of course plays a role in behavior, I think it plays out more in directing the development of the brain, not the actual millisecond-to-millisecond activities of it. Based on my ankle-deep knowledge of neuroscience, I think that adjusting the DNA is unlikely to cause an adult brain to rearrange itself in such a way as to modify behavior. A more likely use for it could be in adjusting brain chemistry, which I'm guessing can have dramatic, if not entirely targeted, effects.