OK, I'm going to be talking about ways that sex changes can/could happen in a story. I'm not saying that this is how you have to do things, but my goal is to look at the actual biology/genetics and try to keep things compatible with reality to the greatest extent possible.

This is what's called a "hard" science fiction approach. You take the way reality works (to the best of our knowledge) and make the minimum changes required to achieve your story's goals.

So, since the first thing you need to know when writing about anything is how things are in the real world. This prevents you from making errors like giving Switzerland a seacoast.

Just ran into another story here where the "new girl" is told how to get into a car.

You know, butt on the seat, legs together, pivot...

Not making a lot of progress on writing. Fighting a cold for the last week or so.

And I've had a few ideas that just won't fit.