In our world everyone has secrets.
In this world, which is like our own, everyone has secrets as well.
But the nature of some secrets in this world is different.
Anybody in this world has a chance to wake up to a change. Usually a small change, sometimes a big change.
This change will be their secret. They will innately know the time they need to keep their secret.
It can be anything from an hour to a year or longer. The impact of the change and the time tend to be inversely proportional, but naturally exceptions make for interesting cases.
If anybody finds out the secret, it will become permanent. Anybody making the connection between the original state and the new state to be result of a secret, will result in permanency.
"Didn't Bob from accounting used to have a beard? He's suddenly clean shaven. Oh, is his secret he doesn't grow facial hair anymore?"
Bob is now unable to grow a beard. It does not have to be said out loud.
If you like the change, or find that hiding it is too much of a hassle, you can of course just come out and tell people. Known as "coming out".
But all parts of a secret have to be included. So if Bob actually doesn't grow facial hair anymore because he turned into a woman, it would not have been enough for the change to become permanent.
Some secrets come in "stages". So maybe Bob started out only unable to grow facial hair, but after somebody guessed that, he wakes up the next morning with the next stage of having boobs and if he does not manage to hid it until it has run it's course, he will completely feminize.
If you don't like your secret you can try to hide it. Wear contacts to hide your changed eye color, that kind of thing.
Everybody knows about the existence of secrets, so guessing a change is due to a secret is very common.
In general secrets and the resulting changes have high societal acceptance. Similarly companies may allow employees to call in "sick" for a short lived secret, so you can just wait it out at home.
Almost always the changes will something the person receiving them would like to hide. Be desperate to hide even. Barbara, a model proud of her tanned dark skin and using it as the basis for her income, is more likely to wake up as a white skinned redhead that can't tan for the life of her, than somebody who is only tanned when they were on holiday and works as a secretary.
This world has developed agencies and companies that have specialized in hiding your secrets for you. Of course, you have to manage to use their services without the employees making the connection of what your secret is. Usually, this is done with the help of "reverse secrets".
Barbara in the example above could call up the agency under a false name and arrange a way to live as a redhead until the secret has run it course.