Would it be theoretically possible to alter someone's, an adult human's for example, Chromosomes and DNA in order to change sex?

The reason this peaks my curiosity is because there are some animals that are capable of completely changing their sex, even after adult hood. This includes genitalia and internal organ structure. I know this isn't genetic but it's still changing the sex of a given individual without something like surgery, it's a natural process.

Is a similar process possible to replicate in Humans and is changing one's genetics the key to that. Would such a theoretical procedure cause an adult human to be able to change sex, genitalia and all, without surgery. Or is such a thing simply not even theoretically possible? What makes humans unable to do what other animals can do naturally.