I have a very rudimentary understanding of genetics (meaning I now almost nothing and don't know what I'm talking about).

But from what I understand with genetics is the blueprint for making a human, and for most genes retroactively changing them after the human is fully grown does nothing in most instances. Exceptions for things like genes that have yet to expresses themselves or would express themselves later in life. But physical appearance cannot be changed.

So in order to actually change your appearance you'd have to not only change your genetics, but re-do the developmental process all over again. Basically rebuilding your body piece by piece.

An example would be that if someone wanted to be a female when they are already an adult male changing their genetics is able to change hormone production, but not change any primary sexual characteristics like genitalia, skeletal structure or facial features.

So I ask if making such drastic post development changes would be possible for humans ever? Other species can make these drastic changes but it seems humans and other mammals can't. What is it that prevents humans to change things like sex as easily as fish can. Is it possible to take a fully grown human and start over?

Sorry if I sound like an idiot but as I stated I don't know what I'm talking about so I'm just positing the questions I have wracking in my brain.