Why would you assume that? That's like saying by the time we can manufacture a better engine we'll be able to replace a running one with the new design.

For example, evolution has optimized and delivered a mechanism for turning gene edits in a fertilized egg into a developed brain. It has not done the same for incorporating after-the-fact edits into an existing brain. So in the adult case we have to do an extra giga-evolve-years of optimization before it works.

I suspect that by the time designer babies become an issue the biotechnology will advance far enough to allow genetic modifications in adults on the fly, including natural sex change

Sex differentiation mostly occurs during embryonic/fetal development. If you were to somehow switch the sex chromosomes of all cells of an adult human, or even a newborn human, it wouldn't magically transform a penis into a vagina or vice versa. All you would get would be to screw that person's endocrine system causing infertility and other health problems.

After a bit of Googling I find that there a company that manage to clone 500 pig per year. In a decade they might try themselves on humans.

Thank god I wasn't the only one who thought of this. There's going to be a 17 year lag between when the babies are designed and when they enter the workforce so lots of time to figure out how to do the same thing in adults. It seems unlikely that we will have very advanced genetic manipulation technology, but that gene therapy will still be in its infancy, and no one will have cybernetic implants.