Altargen
Temporary/Permanent gender change has been made possible through a new technology. This technology has been marketed as the "Altargen Experience", and while generally safe has one major danger - if one in a transformed state becomes pregnant, the change may become permanent. Later stories may feature governmental support of the technology, while in foundation era stories it is still experimental and only available in one place.
If anyone would like to write a story in this universe, please contact me first (for the sake of keeping the universe more or less consistent) so we can discuss details and I can offer any assistance you may need.