Psychosis is a condition of the mind that results in difficulties determining what is real and what is not real
so I think there would be elements of that at first. It would probably feel dream like and you would question reality but I imagine for most people the reality of it would sink in pretty quick. You would still have a sizeable chunk of your life and surroundings to ground you.
This is all likely extremely dependent on someone’s views of sex and gender though.
Even if you’re transgender your body slowly morphs at the chemical level into a woman or a man over a long period of time. A second puberty as I’ve heard it referred to. This would give the mind time to adjust gradually to a new reality/brain/prospective seeing as how one’s brain slowly changes over time on sex hormones.
As you mentioned there’s the brain chemistry aspect but an abrupt change like that on the physical level may be more similar to wearing the stereotypical clothes of the opposite sex and passing off as it. The big change could be how different society treats you.