Genderswap is a fanfiction trope in which the gender of one or more characters is flipped - due to the prevelance of major female characters in Daria, this tends to be female characters swapped to male.
This has become extremely common in Daria fandom, with every single character (yes even Upchuck) swapped at some time or another. This can be for comedy, drama, or as a shorthand for it being an alternate universe.
Origin of the trope
While the trope had been done before and isn't unique to this fandom, the reason it is ubiquitous is Nick Gaston: in early 2003 he briefly mentioned an idea he had for an AU story featuring a Jacob "Jay" Lane, on an incarnation of the Paperpusher's Message Board. TAG replied that he had a similar idea he'd been working on, and that if Nick ever posted the "Jay" story, he'd start on it.
While Gaston gave up on the idea, he left it up for "adoption" - author Beth Ann was able to use it as a basis for her series "Age Old Question", which would lead to TAG's "Darius" and that fanfic would inspire John Lane. These fanfics have gone on to inspire hordes more.
- Darren's Estimation by Bacner, the first one of these ever done. Darren Morgendorffer is a far less ethical figure and up for hire, who'd used Beavis and Butt-head as paid flunkies.
- Age Old Question by Beth Ann: the one that started the post-show genderswap wave. Daria and Jacob Lane are trying to avoid acting on their feelings for each other, while Jacon and Trent have a shattered relationship after Trent stole his brother's girlfriend.
- Darius by TAG: a dark drama fic in which Daria being born male causes Jake to become an abusive parent, reflecting what happened with him, and Darius is struggling to be a good man.
- John Lane by Richard Lobinske: a running serial where the events of the show are increasingly altered due to Jane being a John, starting with a seemingly minor change in Daria's attitude during "The Daria Diaries".
- Finn Morgendorffer by HolyGrail2007: Melodramatic serial where a male Quinn is still as popular and mercenary as in canon, but now a 'ladies man' jock - with a far more toxic relationship with Daria.
- Not So Different by J-D: A realist serial about a male Daria, nicknamed 'Sonny', and the minor differences that build up from the genderswap, as well as his relationship with Tom.