In And Eternity, Orlene is temporarily transformed into a man to make a point.
Several Animorphs characters do cross-gender human morphs. Marco morphs the female governor in one book, and presumably, Cassie and Rachel morph male when they acquire sailors for a mission on an aircraft carrier. Rachel's eagle, acquired between books 1 and 2, is male, as is the grizzly bear that she acquired in book 7. Marco acquires a girl wolf in book 3, while Jake, Marco and Ax all become a female skunk in book 9, and Rachel and Cassie (along with the four boys) acquire a male polar bear in book 25. Tobias acquires Taylor, a human female, in book 43. In The Hork-Bajir Chronicles Aldrea (who is female) morphs Alloran, the future host of the below-mentioned Visser Three.
This is the plot point of one book, where Visser Three morphs into a female human and masquerades as Tobias' cousin Aria in order to capture him after he is read a letter from his father, Prince Elfangor. Visser Three spends most of his time in this form during the book, just in case Tobias is morph-capable and spied on him in animal form; Tobias only figures out the trick because Visser Three is too good at acting like a human woman and checks "her" hair in a mirror, which is automatic for most women but not necessarily for one who's been living in Africa for years, which was what "Aria" was supposed to be doing for most of Tobias' life.
In Judith Tarr's Avaryan Rising series, a young nobleman from one nation is turned female—by way of a dangerous magic ritual—so that s/he can marry the prince of another nation for diplomatic reasons. Predictably, she's smoking hot in female form.
Tanith Lee also explores gender-switching in her Biting the Sun series, where changing bodies and genders is almost as easy as changing clothes. Officially you're only supposed to change once every thirty days, but nobody pays attention to that.
Chakona Space: Skunktaurs, which were a competing project to the chakats' development, can switch between male and female. They're born male then change to female for the first time at puberty and can trigger the change at will from then on.
Gender-bending is a frequent occurrence in many of Jack Chalker's works.
On the Well World, the Well Computer thinks nothing of changing an Entry's sex. In Midnight at the Well of Souls, the very definitely male Datham Hain is changed into a female Akkafian, while Elkinos Skander is changed into a Umiau, a hermaphroditic kind of mermaid.
In the Soul Rider series, many of the antagonists are very misogynistic men. To them, changing a man into a woman is a severe punishment. In later books the men of New Eden take it further: men who commit certain kinds of crimes are transformed into "fluxgirls," very beautiful women who are slightly brain-damaged so they're only fit to be housewives and baby factories, and who are addicted to sex.
In the Four Lords of the Diamond series, the weird aspect of the planet Cerberus is body-switching: two people who sleep in close proximity are like as not to exchange bodies during the night. The planets Charon and Medusa also offer opportunities for gender-changing.
Neil Gaiman's 1998 short story "Changes" has Gender Bending as its premise, as a cancer cure called Reboot that works by altering the patients DNA has the side-effect of transforming the patient into a fully functional and fertile member of the opposite gender. The story takes place over several decades, analyzing how society would react to the effects introduced by such a drug. The middle eastern response is especially horrific, in contrast to the Sex Gates example above.
In Cordwainer Smith's Instrumentality short story, "The Crime and the Glory of Commander Suzdal," the Arachosians made their women into man-like things (klopts) on purpose. It was a Fate Worse than Death.
In Iain M. Banks's Culture novels, citizens of the Culture are able to change their gender at will. The process is slow, taking several months, but is done entirely using consciously-released hormone, not requiring any external assistance. The change is so complete that they begin to produce the appropriate sex cells, and a newly-female person can get pregnant and give birth. Most people in the Culture will spend a year or two as the opposite sex at some point in their lives, just to see what it's like. A popular expression of love is for a couple to conceive a child, then change gender and conceive another, so both partners are pregnant with the other's child at the same time (they are also able to temporarily arrest the development of the embryo/fetus, so that they can time it so they come to term together — if the embryo is only just conceived, they can even keep it alive while they become male).
In the novel Cycler by Lauren McLaughlin, the main character Jill changes into a boy, Jack, before (or during) her period.
Demon Sword Maiden starts the story with a very, very effeminate boy getting confessed to by his older, taller, and very busty crush. He reciprocates, and then the plane they're flying in crashes, killing them both. The next moment, he wakes up in her body, while her soul is stuck in a magic mirror. The prevailing conflict in the story is him trying to find a way to get her out and give her her body back, becoming a powerful demon-slaying samurai in the process...
Doom Valley Prep School has the main character, who is happily male, get turned into an ideal girlfriend by an alchemist on the way to school. There was a 40% chance it would go wrong with horrible results, so it wasn't exactly safe, but it was quick only taking a few hours. There are spells and potions to turn her back into him, but the school doesn't want to spend the money on it, and it's too expensive for her to pay for it. Most people who learn of the change aren't bothered by it, but she really dislikes it.
In Gael Baudino's Dragon trilogy, an enemy wizard does this to an entire troop of soldiers from an extremely sexist country. The transformation is permanent, and the characters' adaptation to their new state is dealt with at length; the new women's situations are dealt with surprising sensitivity, given that at the time Baudino was a Dianic Wiccan.
In the Dune series, the Tleilaxu Face Dancers were shape-shifters with the ability to change gender.
The middle part of The Elminster Series: The Making of a Mage is basically "The Making of a Priestess". When Elminster agreed to serve the goddess of magic, he finds himself teleported without any warning to an unknown location and turned into a lass-"to see the world through the eyes of a woman" (and to hide him from Magelords out for his blood), having nothing on her save half of a broken ancestral sword, and no magic abilities save innate magic vision. Later he learned the spell himself. In the next book, Mystra pushes a silent sex-inverting spell into his mind, to give him a way to circumvent controls built into his body. Which he used while embraced... er... restrained by his current Love Interest, no less. "Well met. Call me Elmara, please!"
Ian Covey, the doppelganger in Everyman takes the form of two different women.
The Four Profound Weaves: The first of the titular magical arts weaves the wind into a "carpet of change", which can transform a person's body to match their inner self. Transgender people in the desert nomads commonly use these when they transition: one of the protagonists did as a young child, while the other kept his carpet for forty years and finally used it in his sixties.
A rather awkward version happens in Galaxy of Fear: The Brain Spiders. Jabba the Hutt has a weird gambit going where he removes the brains of major criminals, turns their bodies in for the reward money, and installs their brains into the bodies of hapless people in his fortress - their brains end up in spider droids, which sucks. A particularly large and brutish killer's intended body is set free, so he gets put into Tash Arranda, a not-yet-fourteen-year-old girl. At first he doesn't mind much, but soon he starts complaining to Jabba about it. Jabba is just amused. The narration, once the point of view character figures it out, calls him "he".
In Garrett, P.I.'s Dread Brass Shadows, Garrett encounters a couple of characters who magically disguise themselves as other people, sometimes of the opposite sex. Finding the corpse of one of them, he's squicked out to discover that the dead man is reverting from his assumed female form to his natural one only gradually, from top to bottom.
An important point in Charles Stross' Glasshouse is that the characters come from a society where Gender Benders are ridiculously easy. It's implied that most people tend to settle on a particular gender for the sake of convenience; the protagonist is usually male, but doesn't even remember what his original body's gender was.
The Harem Protagonist Was Turned Into A Girl!! And Doesn’t Want To Change Back!!!??: This was what alien Mynx intended from the the Issiod’rian Sexual Reassignment ray - to try and punish "Kevin" and with forced transformation and make Plynx no longer interested in "him". She fails on both counts as Plynx is bisexual and "Kevin" immediately experiences gender euphoria once transformed and begins to socially transition more or less immediately quickly changing her name to Svetlana.
Polyjuice Potion in the Harry Potter series has been stated to do this, although we never actually learn how far the gender bending goes. Some examples in the series:
In Goblet of Fire, Barty Crouch, Jr. escaped Azkaban by swapping identities with his mother via Polyjuice Potion.
In Half-Blood Prince, Crabbe and Goyle take the Polyjuice Potion many times to pose as first year girls, in order to allow Malfoy to repair the Vanishing Cabinet.
In Deathly Hallows, in order to get Harry out of Privet Drive, five of his friends (and Mundungus Fletcher) disguise themselves as Harry using Polyjuice potion. Two of the decoy Harrys are Hermione and Fleur. Amusingly, Fleur isn't too happy about being Harry; after the transformation is complete, she tells Bill not to look at her because she's hideous.
In the 1952 French short story Héloïse by Marcel Aymé, the thirty-five-year old Martin turns into the title female character each evening, causing her to clash with his wife. Things get complicated when Martin falls in love with Héloïse, and when Héloïse gets out and sleeps with other men. At the end of the story, Martin's wife tells him that Héloïse is pregnant; and that Martin/Héloïse will give birth to a boy who will soon turn into a girl.
Women's Need Calls Me, a short story in the Heralds of Valdemar setting, features the magic sword Need, generally believed to be Gender-Restricted Gear, who chooses a trans woman with intense dysphoria. In a later-set book Need is able to make small and greatly desired changes, day by day, to another character's body which add up to a clear difference several months later. Here, Need just dismantles an ominous locus of magic energy and uses it to completely transform Pol's body overnight, though doing so came with some risk of failure and drains the sword into 'sleeping' for decades.
K. Lynn's His Womanly Ways employs the body-horror route with a gradual genderswap for the main character. Womanizer gets cursed to know what it's like for a woman, and he slowly starts turning into one. The change is found to be permanent, but the novel subverts the typical genderswap tropes by utilizing an underlying transgender theme.
In the Mercedes Lackey and Piers Anthony novel, If I Pay Thee Not In Gold, there be demons. They look ordinary, if beautiful, and with awesome eyes, but normal for normals where they come from. What distinguishes them is immortality... and gender bending when they have sex. Imagine losing your virginity and learning that. How can demons die? By being killed, or if a partner is unfaithful.
In I'm In Love With the Villainess, Prince Yuu suffers from this after he was accidentally infected with a gender-bending disease when he was young. Complicating matters is that he wants to return to his birth gender, but his mother doesn't want him to endanger his chances at the throne. He eventually cures himself of the disease, and happily lives on as a female.
In Kämpfer, Natsuru Senou, the (originally) male lead character, transforms into a girl whenever he becomes a Kämpfer whether he wants to or not.
Princess Ozma of L. Frank Baum's Land of Oz series was magically changed into a boy as a baby, and grew up this way (under the name "Tip") until the spell was discovered and she was changed back. The fact that their personalities are total opposites has led to at least a century of many fans trying to explain this away via or Retcon. Lampshaded when the Gump mentions he thought Ozma was nicer when she was a boy. The Gump also mentions that Ozma didn't much like him talking since she changed her name.
When Kali is asked to become the new Brahma, she agrees, even though it means breaking off her marriage to Yama in the middle of their honeymoon. She is later reincarnated as a female again, under the name Murga, who Yama introduces to Kubera as his daughter.
This is the ending twist of the Clive Barker short story "The Madonna" from The Books of Blood. All men who enter The Madonna's lair become women. Though it can be terrifying for some people, he/she ended up better than mostother characters.
Magical Girl Policy: Rob is revealed to be Spirit Guard Serenity, and will be permanently transformed into a girl upon transforming the first time.
In Magical Girl Raising Project, no matter what you look like in real life, your magical girl form will be an idealized version of you as a magical girl, and will always be female. Naturally, not many people in the cast realizes that La Pucelle is actually male in human form.
Magic University: People can change sex with magic here. Brandish was female, male, then female again. Alex is not sure whether she'd been (physically) female originally or not. It turns out she's a trans woman, transitioning this way physically.
One of the titular Midnight's Children can do this by immersing themselves in water.
The Japanese novel Mister Tsubakiyama's Seven Days depicts the death of the eponymous middle-aged salaryman. However, he believes that he still needs more time to see his family off before ascending to the afterlife, and as such is given the opportunity to see them again for seven days, temporarily in the guise of a beautiful woman. This story was adapted into several TV films and even a Korean TV show remake.
For the people of Greg Egan's novella Oceanic, who otherwise seem mostly human, coïtus ends with the penis transferring itself to the other partner. The protagonist has one sibling; each of their parents is mother to one and father to the other. (The people's origin legend suggests that their ancestors had been disembodied software for a long time before creating their bodies.)
In Peter Pan in Scarlet, the former Lost Boys must wear their children's clothing in order to become children again and return to Neverland. Unfortunately for Tootles, he has no sons, so must wear his daughter's ballet dress, and turns into a girl.
"PMS Zombies" shows the troubles of a teenage boy unwillingly turning into a beautiful girl right as all the women in town turn into man-hungry cannibal zombies.
Pojkarna by Jessica Schiefauer - three teen girls who suffer from misogynistic bullying at school find a mysterious seed that grows into a flower that can turn them into boys if they drink its nectar, though it wears off by morning. One of the teens is heavily hinted to be a trans man and finds the situation liberating and becomes addicted to the nectar, whilst the other two are intrigued and find some more confidence as boys admit they'd rather stay as girls.
In Princess Holy Aura, a 35-year-old man's choice to become a 14-year-old girl is one of the key points of the story.
The Queen of Ieflaria: A common magic in the setting, and why no one is too worried about the heir of two women. The problem is that you have to want the Change for it to stick; actual trans people are easily able to Change permanently, but there is some doubt whether either Esofi or Adale will be able to maintain it long enough to produce an heir.
In the Safehold series by David Weber, Nimue Alban (or, specifically, the robot with Nimue Alban's personality) crafts the persona of Merlin Athrawes. Her robotic body has the capability to shift genders, which she uses to complete the image.
The premise of the Sex Gates trilogy by Darrell Bain & Jeanine Berry: Portals appear at random all over the world. If they don't make you vanish when you pass through them, they cause you to emerge as a flawlessly healthy 18-year-old... of the opposite gender. Hilarity Ensues. Well, not so much hilarity as extreme societal upheaval. The Middle Eastern response — namely, chucking various imams and abusive husbands through, was highlyamusing. Other aspects, like the Trans Equals Gay implications of homosexuals lining up for their Gender Bender or women eager for the chance to be the "superior" sex, may be less amusing.
In A Song of Ice and Fire dragons are described by Maester Aemon as "neither male nor female, [...] but now one and now the other, as changeable as flame." His knowledge of dragons being able change sex at will comes from a rare book from scholar Septon Barth, who also served as Hand to Jaehaerys I. Interestingly during the books several dragons are referred to by a specific gender, for instance Balerion is referred to as male by Daenerys and Tyrion refers to Syrax as a "she-dragon", suggesting some dragons may remain a single fixed gender.
In Sphere, the protagonist tests out his reality-warping abilities on some nearby mummified corpses, changing men to women and women to men. That's right: gender-bending corpses.
In Stardust, Bernard the goat herder is first turned into a goat by the Big Bad, and later into a woman to pose as her daughter.
Alison Tyler goes the typical bodyswap route in Tiffany Twisted. A woman and her boyfriend switch bodies and explore each other in intimate and personal ways. Of course, the male-turned-female character uses the opportunity to feel himself up. The point of the swap in this case is to teach the protagonists a lesson — namely, to be more respectful and considerate of each other.
Torchwood: In "Almost Perfect'', the male Ianto is given the form of the dead woman Christine through the Perfection's machine. Some exploration is made of the reality of Ianto becoming a woman, such as being sexualised, before he starts gaining stereotypically feminine traits such as cleaning and watching his weight.
In Turnabout (1931) by Thorne Smith, an arguing married couple have their minds switched between bodies by a magical Egyptian statue in their bedroom. This book was the basis of the 1940 movie and 1979 television series by the same name, which was turned into the movie The Magic Statue, and the book was more loosely the basis for the movies Dating the Enemy, It's a Boy Girl Thing, and The Swap. Despite being a written format, the novel incorporates Voices Are Mental into the story line. During the swap, the man's voice comes out of the woman's mouth, and vice versa.
In the Vampire: The Masquerade series, Sasch Vykos was once Myca Vykos, but changed genders via Vicissitude.
A Civil Campaign has, as a significant character, Lord Dono Vorrutyer... formerly Lady Donna Vorrutyer.
Not to mention Bel Thorne the hermaphrodite, who, while technically both male and female, is quite fond of switching its perceived gender around just to mess with people's heads.
In Warrior Cats, the others often can't keep up with the massive cast, especially characters who were created just to flesh out the allegiances, so this has happened to several characters (most notably Sedgewhisker and Rowanclaw).
Whateley Universe: This is a major point of many of the tales, due to them taking place in an LGBI dormitory at the eponymous Superhero School.
Several of the main characters are gender bent by their "Exemplar" mutant power, which involves Involuntary Shapeshifting to match their "BIT" (Body Image Template). A BIT is that person's mental image of a perfect body, which is occasionally the "wrong" gender — although Exemplar mutations tend to have a side effect of a certain level of acceptance, and many were transgender to begin with (i.e., the changes fit what they wanted to be all along). Note that a BIT can lead to an intersexed state as well — this is -the case with both Phase and Circuit Breaker - or cause a non-human appearance. It should also be noted that the First Law is in full effect in the Whateley 'verse — as a rule, it is not possible to undo or really change an Exemplar mutation.
Other than the Exemplar mutants, one main character is changed to a girl by being bonded with a powerful female elf spirit, another due to a poorly worded prank by her little brother, and yet another hasn't changed at all but is actually a transgender girl trying to find a way to transition. Oh, and then there's Hank, who's gender bending the other way.
And that's not even counting the ways it has happened separate from mutations. These include finding and bonding with a magic sword (Bladedancer); deliberate magic (Scapegrace); botched magic (the Crystal Wavers); getting affected by a booby-trapped book meant to protect her from a familial curse (Josie Gilman); conning a horny Mad Scientist into doing it (Delta Spike); a Mad Scientistaccidentally dosing himself with his own 'perfect Drow girlfriend' transformation serum (Jobe Wilkins); memory transfer of a copy of a male Mad Scientist's self into a female Drow clone body (Belphoebe); getting dosed with a sex-slave serum (Daphne Han, who then turned the tables on her captor); bonding with powerful spirit beings (several of the Loose Cannons); making a poorly-worded wish with a Power Gem (Hat Trick); being an unwilling subject of magical experiments (Dragonsfyre); getting affected by nanotech (Tyler Collier); getting affected by nanotech and a magic-based computer game (Whisper); and getting zapped by an angry Mad Scientist girlfriend (both Reach and Lapin).
Things go completely off the rails in the story "Smoke and Mirrors"[1], when someone finds a devise called a 'Mimicry Inducer' (which imposes someone else's appearance on a person), and decides to use it as an offensive weapon. Fortunately, the First Law of Gender Bending doesn't apply for once, though at least one permanent male-to-female change does occur (to someone who happened to be Transgender to begin with, so no one was complaining).
It should be added that the rate at which the transformations take place can vary greatly, from Tennyo's near-instantaneous self-immolation/transformation, to the half-decade long gradual self-adjustment that Mega-Girl is going through. In between that are Crimson Comet (three days), Phase (about a week), Shadowdancer (two months of agonizing illness), Fey (six months of slow changes followed by a sudden, unpleasant burst of alterations after her first major use of her powers), Chaka (about a year, and requiring surgical intervention to correct her hips half-way through), and Beltane (a couple of years of self-modification). And that's not counting shapeshifters like Jimmy T., who can voluntarily change form in a short period, or those who transform involuntarily more or less instantaneously at varying times, such as Reach, Chimera, Fling, or Heyoka.
Xanadu (Storyverse): A number of transformations cause people to change genders and/or physical sexes, such as a frat guy who wore an Ariel costume on a dare and is turned into a mermaid, a man turned into a female Fox Folk, a woman who is turned into a Hulk Hogan clone, and two women who were part of a group cosplaying as marines from Hogan's Heroes. The main character in "Refamiliarization" also undergoes this as his full transformation slowly sets in, but the fact that he's turning into a female dragon specifically means that the sex change is largely a secondary issue for her.
In Crewel Lye, Jordan has a bag full of counter-spells for various magic traps he's meant to encounter on his adventure, but they were tampered with and have the wrong effect. When he tries using one of the spells, he ends up switching bodies with his companion Threnody (although this is a stroke of luck — Jordan notes the spell was probably supposed to switch him with his horse instead). The two grow much closer while figuring out how to use their new bodies and Talents to escape. They end up triggering another trap later on that conveniently causes them to switch back, which is what the initial swap was supposed to be a counter for.
At one point in Pet Peeve, Goody and Hannah wander into an area called No Man's Land, which causes any men who enter it to permanently become women. After some teasing from Hannah and Peeve, they travel through No Man's Land for a way to reverse it. In the center is a jewel that when flipped over causes the area to become No Woman's Land, which has the opposite effect. The inhabitants of No(Wo)man's Land apparently don't notice the change, and it doesn't seem to have any effect on the Pet Peeve either. After turning back into a man, Goody leaves so Hannah can flip it again to get herself back to normal.
shows the troubles of a teenage boy unwillingly turning into a beautiful girl right as all the women in town turn into man-hungry cannibal zombies.
