Somewhere in the multiverse, there is a version of our world that is exactly like ours with the exception of one very strange twist.
After the age of 18, at any point in their lives, about 5% of the population undergoes something that has come to be called Upside-Down Transformation, or UDTF for short.
In a visceral, surreal process that takes a few minutes or up to an hour, arms and hands become legs and feet, and vice versa. Shoulders become hips, and hips become shoulders. A new ribcage forms where the abdomen used to be, while the old one vanishes. Breasts, if there, might travel to a new location or disappear entirely. Most importantly of all, the person's head becomes genitalia, while their genitals become a new head that assumes control of the body. Sometimes the old body leaves a trace of its existence behind, like a penis of a different skin tone or pubic hair of a different color.
The new person born of UDTF has all the old person's memories, but their personality might be similar or vastly different. As for the previous owner of the body, people newly formed by UDTF can still hear and communicate with them; their personality is largely dormant and primarily concerned with what genitals usually do. They never quite leave, but their body is never theirs again. This "dual mind" makes the UDTFd valued as intelligent multitaskers.
UDTF never happens without warning. Up to a few years ahead of the event, the new personality awakens, first as a whispering voice in the host's subconscious, then slowly getting stronger and more dominant. Soon the host realizes their genitals are talking to them. As mentioned before, this new personality can be similar to the host's or very different; it is created from the host's subconscious desires. A shy person might create an assertive, brash personality who eventually takes over and flips them around, but it is just as possible that an extroverted party animal would create a shy, mousy personality who apologizes as they take over their body.
Men and women are equally likely to UDTF. In fact, 50% of UDTFs are gender swaps. Some of those whose gender changes upon UDTF exhibit gender dysphoria, but others do not. There is also seemingly no rule governing the result of the transformation. Someone can UDTF into someone else who is very similar, but a lot of times gender, ethnicity, body type, height, weight, and hair and eye color can change drastically. An obese Asian man can UDTF into a skinny black woman, and a cute redhead can find herself bulking up as her head turns into the thick cock of a bulky, hairy man. Research is still under way to determine how this happens, but the generally held theory is that these vastly different people are born from the host wishing they were something different.
There are, of course, plenty of aids for future UDTFers and those that have already UDTFd, ranging from bestselling books to support groups. They face some stigma in society; a woman whose host used to be a man might get nastily referred to as "Miss Dick" before others get used to her. And that's not going into what men who UDTFd from women face. In general, those whose gender does not change after UDTF are treated better. Not to mention that those who UDTF after having a family might get kicked out by their spouses and children, unable to accept the new person suddenly living with them. Fortunately, pregnancy puts UDTF on hold, but the mother can expect to suddenly UDTF right after giving birth. UDTFd men and women are perfectly normal and can reproduce normally.