Genderswap as a term actually covers several different fandom tropes. Genderswap is a subset of the more general genderfuck type of fanworks, and stories can include several different scenarios. In one, a "swap" of the character's sex is usually carried out by some magical or technological device, making the trope popular in sci-fi or fantasy stories. In most of these genderswap stories, the character is restored to their original sex by the end, although some stories consciously subvert this trope. Alternatively, some genderswap fic and most genderswap art involves depicting a binary-gendered character as a cis person of a different gender.
Star Trek had a genderswap (actually a bodyswap, too) in canon in the final episode, "Turnabout Intruder", and in one of the first collections of professionally distributed fiction, The Procrustean Petard in Star Trek: The New Voyages 2 (1978). Despite this, genderswap stories have only become hugely popular in the Internet age, and are much easier to find in fandoms like popslash, Stargate Atlantis, Supernatural and Star Trek reboot than in Classic Star Trek, The Professionals or Blake's 7.
Genderswap is not the universal fandom term for this trope, and there are a number of terms fans use to describe the trope and its subtropes and related tropes.
- Genderswap - used for two tropes: 1) in-story sudden (magical or sfnal) gender/sex change and 2) always-a-different-gender
- Genderbend - sometimes a synonym of genderswap, sometimes a synonym of genderfuck, sometimes refers to miscellaneous gender play not covered by a more specific term
- Genderfuck - overarching term for fanwork tropes that play with gender
- Gender Flip - TV Tropes article on recasting characters as a different gender (e.g. Starbuck in the BSG reboot, Pidge in Voltron: Legendary Defender)
- Gender Bender - TV Tropes article on in-story sudden gender/sex change
- Rule 63 - general internet term that is sometimes used for the always-a-girl trope. The original Rule 63 has a slightly different meaning. See Urban Dictionary.
The term 'genderswap' itself, as well as the way it most often plays out in fanworks, has been criticised for falsely assuming that both sex and gender are binaries, and for implying that sex and gender necessarily coincide. Various alternatives have been proposed, such as spectrum slide. genderbending has caught on in the 2010s. See this August 2017 discussion, Archived version for more about the history of terms, usage, and some current issues.
While some assert that genderswap should correctly be called sexswap, since what is being changed are the visible sex characteristics and the social gender only as a consequence of the physical change, the term is established for a number of likely reasons, e.g. what some name the conflation of sex and gender in discourse or the better flow of words.
A similar criticism of both the practice and its name doesn't focus as much on the distinction between sex and gender, because sex is seen as just as socially constructed as gender. Rather, it focuses on the particular way bodies and gender are assumed to be related in these kind of stories, highlighting how genderswaps often erase or oversimplify experiences of trans people with their bodies. Cisswap is a term often used in this context for genderswaps which assume that having a body that society assigns a different gender to would lead to the character actually being that gender, or that reinforces cissexist norms in some other way.
Further pointing to the fact that the gender is not what has been changed, in the large majority of genderswap stories, the character who has been swapped as part of the plot retains their pronouns in the (generally) third person narrative.
Always Been a Girl
A variation on the genderswap trope is "always been a girl" AUs or "Rule 63" fic, where one or more of the canon characters are written as if they have always had a different gender from the one in canon. These stories tend less towards crackfic and more towards seeing what might have changed in a favored character if this one piece of their life had been changed. In fandoms where alternate realities exist in canon, these sorts of AUs seem almost inevitable. However, in the show Sliders, where the entire premise of the show was based on traveling to alternate realities, they only once encountered a sex-swapped version of one of their team. In fandoms such as Supernatural and SGA, where the main action heroes are all men, the use of the trope to write the universe from the point of view of a cisgender woman is popular. Rewriting female characters (such as Teyla) as men is very rare.
With the release of the Star Trek reboot movie in 2009, there has been a surge of interest in genderswap fic in which one, or more, of the characters — almost exclusively male — has always been a different sex from what s/he is in canon. Kirk appears to be the character most often portrayed as female, with McCoy, Spock and Chekov trailing well behind with less than half the number of instances. This offshoot of the reboot fandom is popular enough to have launched the LiveJournal community st_genderswap, two months after the movie's release.
Naming and Labeling
These stories are often tagged or labeled with the prefix Girl, such as Girl!Jim/Spock or Girl!John/Girl!Rodney to differentiate them from non-genderswapped versions. Similarly "fem!" and "male!" may be used to label stories that feature a character rewritten as female and male respectively. There is some controversy in fandom as to whether a pairing is het or femslash if you've made one or both of a popular slash pairing female, either from birth or from some magical change. If a story involves a temporary change, and gender is therefore not altered, most fans will consider the change to have no bearing on the classification. If a story is an always-a-girl, or always-a-boy, opinions vary more widely.
With always-a-girl genderswap fics, the author is essentially creating a new character, and their new character needs a name. Popular choices for "girls'" names include canon references if available, like Meredith McKay or Deana Winchester, or standard feminized versions of the original name, like Johanna or Joan Watson, Harriet Potter, etc. In some cases, writers simply use the original name, either without explanation or as a nickname or short form of a more standard female-sounding name. Rarely is a totally unrelated name chosen, reinforcing the idea that the swapped character is intended to be the same person, not just a child of a different sex born to the same parents.
Controversy and Criticism of the Trope
A 2010 comment:
[T]oo often I will see the author switch into new pronouns to match the new body, or depict the character enthusiastically "trying out" their new body, often sexually. I can tell you for sure, suddenly having the wrong body means CRAZY issues. I can't even look at myself undressed. A new body means learning to use it, first of all, and then all the dysphoria that comes with it, and one thing it isn't going to do is turn its owner on. Long story short, most "genderswap" (which should really be called "sexswap", considering the context the term is most often used in) offends me hugely, and manages to avoid all the opportunities to bring trans issues into the mix. 
Genderswap is also commonly criticized for focusing on cissexist and often misogynist stereotypes of perceived gender differences, and for having characters immediately adopt a conservative gender performance without question. For example, men having to learn how to shave their legs when their bodies change, even though they could just as easily choose not to, is a common trope in stories where men's bodies are "switched".
Notable Genderswap Fanfiction
Always a Girl (or Boy)
- The Pumpkin Juice Incident by Naadi, Halloween-themed poem and art, Harry/Draco, both magically 'swapped 
- Girl-Sirius by lizardspots, Sirius is genderswapped as a result of a potions accident.
- As Her Highness Commands by lizardspots, girl!Arthur/Merlin (NSFW).
- Merlin's magic another epic fail by rnng. Merlin has accidentally turned himself into a woman and Arthur tries very hard not to watch.
- Genderswap: Ash Ketchum by Y-Mangaka, a play on the ability to chose the playable character's gender in the Pokémon video games.
- The Adventures of Shirley Holmes by Johanna Watson by Mutecornett, one of several recent art (and fic) examples of always-a-girl Holmes and Watson. This one is based on the Ritchie film.
- All turned around by ileliberte, with Kirk, Spock and Uhura genderswapped
- The L-Word: Starfleet Edition by oxboxer, with Kirk, Spock, Sulu, Scotty, Chekov and McCoy genderswapped
- untitled, by rotenschal, Bofur and female!Bilbo Baggins on ponies in the rain.
A lot of cosplay involves genderswap as cosplayers don't limit themselves to characters of the same sex. Cosplaying a character whose gender is different than one's own is called crossplay. Some differentiate between crossplay and genderswapped cosplay, such as "femme" versions of male characters.
- Gender Bent Justice League (LA Weekly)
- Genderswap And Other Alternative Cosplay From Comic-Con 2012 (The Mary Sue)
- Holmes and Watson cosplay by MigraineSky
Part of the trend of recasting picspams, where photos of actors of different races, ethnicities, genders or sex are assembled to "recast" a show.
Male!Harley Quinn by Zeilyan
- Genderswap tag on AO3
- Alternate Universe - Always a Different Sex tag on AO3
- Genderbending tag on AO3
- ninjapikachu215's Naruto Genderswap favorites on DeviantArt
- Bandom: Masterlist: Genderswap by stepps (2008)
- Harry Potter: Themed list: genderbending by painless_j (2008)
- Sherlock: Sherlock Holmes Genderswap Recs (fem!Sherlock) by shadowfireflame (2011)
- Stargate Atlantis: Sheppard/McKay list by monanotlisa (2005)
- Star Trek Reboot: Reclist: Star Trek:2009 - Trope: Genderswap by csad (2011)
- Supernatural, Dean/Castiel: Epic Dean/Castiel AU Fic List: Genderswap/Genderbent 'Verses by swing_set13 (2010)
- Supernatural, Sam/Dean: Genderswap: Born This Way and Genderswap: The Cursed Edition at sd_recs (2011)
- Teen Wolf, Derek/Stiles: jen’s genderswap sterek fic recs (2013)
- spngenderswap (deleted)
- genderfuck - "dedicated to the art of genderbending. Fanart and cosplay will be posted, with majority of content animanga/video game related."
- hellyeahgenderswap - "a place for genderswapped casts of films and tv shows."
- Character X just turned into a girl!: On genderswitch stories by Angela Griffin (2005)
- Bad idea all around. And I'm too irate to cut-tag. archive link by fiercelydreamed (2006)
- An Open Rant Regarding The Genre Of "Genderswap," Specifically Referring To Supernatural And Weemo Bandslash Fandoms by thepurpleswitch (2007)
- Three Things I Wish People Would Bear In Mind When Writing Genderswap archive link page one archive link page two by wychwood (January 2007)
- gender and sexuality in three SGA stories archive link by cathexys for The Cutting Board (February 2007)
- "The Seven Basic Plots," Feminism and Fandom by emily shore archive link by Naraht (February 2007)
- Of the blend of feminine and masculine in slash archive link by paceus (February 2007)
- Fanart meta: genderswap, crossdressing, and trans-ness (2010)
- I’m not even much of a fan of genderbends but goddamn am I even less of a fan of getting ordered around...; archive link and further discussion (2018)
- Spectrum Sliding by Allison "Mu" Jones (NSFW) (Accessed September 23, 2014)
- August 26th 2008. Sherrold. why ask why? LiveJournal post. Accessed October 3rd 2008.
- August 27th 2008. See strangerian's and omnivorously's comments to sherrold's post. Accessed October 3rd 2008.
- cf. August 28th 2008 ifyourweremine's comment to sherrold's post. Accessed October 3rd 2008.
- August 28th 2015. thedeadflag
- Tags for the LJ st_genderswap community record only one instance each of Uhura, Chapel and Saavik as always male, compared to 49 instances of Kirk as always female (as of July 2010).
- As of July 2010, st_genderswap showed 25,22 and 10 instances of always-a-girl for McCoy, Spock and Chekov respectively.
- st_genderswap had over 170 members, as of July 2010.
- iambickilometer. META: Five+ Ways Being Transgender in Fandom Really Sucks, and Why I Stick With It Anyway, posted to Dreamwidth 2010-04-07. (Accessed April 9, 2010)
- Siilie's art blog.
- naadi - WebCite.
- ileliberte, WebCite
- Gender-swapped Doctors are our new favorite form of Doctor Who cosplay by Charlie Jane Anders at io9, 23 February 2012. (Accessed 07 January 2018.)