Common tropes in sex - swap sexy fantasy stories include the transformation being related to a magical object. For example, touching a certain gem might cause the sex - swap. Another is the idea of a prophecy or curse that involves the sex - swap. This can add an air of mystery to the story. And then there's the trope of the character who initially resists the sex - swap but then comes to accept it and learn from the experience, perhaps becoming a better person or a more understanding character in the process.
One common trope is the accidental sex - swap. Maybe it's the result of a magic spell gone wrong. Another is the body - swap for a limited time, like 24 hours, which creates a sense of urgency to reverse it. There's also the trope where the sex - swap is used as a form of punishment or lesson for a character. For example, a misogynistic male character gets swapped into a female body to learn what it's like to face discrimination.
In sexy fantasy stories with sex - swap, a frequent trope is the exploration of sexual attraction. When characters swap sexes, they may find themselves attracted to their former selves or to other characters in new ways. There's also the trope of the fish - out - of - water situation. A character who is used to one gender's body and social norms suddenly has to adapt to the other. And often, there's a search for a way to reverse the sex - swap, which drives much of the story's action.