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.