TS!Underswap (stylized as TS!UNDERSWAP), or Team Switched's Underswap, is an UNDERTALE fangame created by Team Switched for their interpretation of the popular role-swap AU, Underswap. It publicly began development on February 16, 2016  using GameMaker: Studio, and now uses GameMaker: Studio 2 since late 2019.
Characters' roles have been swapped in pairs, much like Underswap. However, this interpretation preserves the original personalities of each character, while expanding upon their roles to a great extent.
While the fangame was originally based on classic Underswap, the team gradually developed a dissatisfaction for the modified personalities of the swapped characters. In order to accurately portray the characters in their new roles, the team decided to develop a much more original interpretation.
TS!Underswap plays identical to UNDERTALE, where the player controls a human as they navigate through the Underground. In the overworld segments, there are various puzzles, interactable encounters, and newly-added overworld dodging mechanics inspired by those of DELTARUNE. Battle segments consist of a bullet hell format, where the player must control a SOUL to dodge monsters' attack patterns.
- Minor details have been added to the game based on unused content from Toby Fox's UNDERTALE (including scrapped concepts, designs, artwork, sprites, and music.)
- Chara takes the role of Frisk. After climbing Mt. Ebott for unknown reasons, this human child fell into the Underground. They are the main protagonist. For unknown reasons, their words will sometimes change...
- Temmie takes the role of Flowey. They are a seemingly abnormal cat/dog monster who encounters Chara at the entrance to the Underground. It is later revealed that they are only one of many, who form a hive mind of sorts.
- Asgore takes the role of Toriel. He is an old monster who acts as the caretaker of Ruined Home, providing guidance and assistance to Chara throughout the area.
- Mad Dummy takes the role of the Dummy. They are a vindictive ghost monster possessing a training dummy, and act as the unwilling tutorial enemy for Chara, and a consistent antagonist throughout the Ruined Home arc.
- Mettacrit (Mettaton) takes the role of Napstablook. He is an attention-loving ghost monster, known throughout Ruined Home as a star. He can be found performing for his adoring fans in The City of Old.
- Papyrus takes the role of Sans. He is an energetic, talented skeleton monster who supervises his brother's antics as a "superhero" from behind the scenes.
- Sans takes the role of Papyrus. Under the name Crossbones, he acts as a vigilance "superhero" who seeks to capture Chara throughout Starlight Isles. When off-duty, he is very laid-back and not very serious. On the genocide route, he will prove himself to actually be very serious, and threaten Chara with a bad time.
- Asriel takes the role of Monster Kid. He is an aspiring artist and creator of the official Crossbones Comics. He goes by the pseudonym "A.S. Dreamer." 
- Flowey takes the role of Temmie. There's only one of him, unlike the Temmies of Undertale. He rescues Chara from the Temmie horde at the end of Ruined Home.
- Alphys takes the role of Undyne. As Admiral of the Royal Navy, she delivers powerful speeches to the residents of Crystal Springs. 
- Undyne takes the role of Alphys. She is the Royal Scientist and owner of the royally-sponsored company Undying Robotics. She created Sir Scratchalot. 
- Sir Scratchalot (Napstablook) takes the role of Mettaton. He's the Underground's top musician and DJ, with his own radio show sponsored by Undying Robotics.
- Toriel takes the role of Asgore. She is the queen of monsterkind.
- Monster Kid takes the role of Asriel.
- Frisk takes the role of Chara.
- Bugerpant (Burgerpants) takes the role of Nice Cream Guy. He acts as the sole delivery boy for every Grillby's location in the Underground. He has very few similarities to the original Nice Cream Guy. He is in love with Flara, causing The Greasers to hate him.
- Muffet takes the role of Grillby. She's the leader of the Spider Scouts, who hold bake sales in Stardust Woods to raise funds for spiders. 
- The Dummy takes the role of Mad Dummy.
- Gaster takes the role of the River Person. He was the former Royal Scientist. 
- River Person takes the role of Gaster. Not much is known regarding this mysterious being.
- Grillby takes the role of Muffet. He owns the retro-themed fast food diner chain, Grillby's.
- Bratty and Catty take the roles of RG01 and RG02.
- BG01 and BG02 take the roles of Bratty and Catty.
- While the fine details of their roles are undecided, it is known that the "BG" stands for either/both "Body Guard" or "Bouncer Guy."
- Syrose and Butchy are two original randomly-encountered enemies found in Ruined Home. 
- Beenard is an original randomly encountered enemy found in Stardust Woods.
- Mordax is an original randomly encountered enemy found in Stardust Woods.
- The Greasers (Aaxel, Norman, Moldrick) are a trio of bullies based off of existing monsters from Undertale.
- Spooky Seltzer and Woodwork are two original randomly-encountered enemies from an older iteration of TS!Underswap, planned to be used for its current iteration. 
- Koffin-K is the count of Koffin Keep, and Crossbones's arch-nemesis.
- His personality is confirmed to take inspiration from those of The Snatcher from A Hat in Time and Antasma from Mario & Luigi: Dream Team.
- Spep is the mascot of Team Switched and appears alongside the Annoying Dogs for shenanigans. His catchphrase is "Put some pep in your step!".
In order to create "new" areas, the seasons corresponding to each major location of Undertale have been swapped, and brand new sub-locations have been created. Locations include:
- Ruined Home (Spring) replaces The Ruins (Autumn)
- Starlight Isles (Summer) replaces Snowdin (Winter)
- Crystal Springs (Autumn) replaces Waterfall (Spring)
- Boreal Bluffs (Winter) replaces Hotland (Summer)
- The NEXUS replaces The CORE
- The City of New replaces New Home
- Minor improvements to UNDERTALE's game mechanics (DELTARUNE-inspired inventory system and overworld dodging mechanics.)
- Brand new accessibility options (official translations, screen resolution, optional aesthetic borders, configurable controls, volume sliders, low VFX settings, and more.)
- The Journal is a new addition which provides a record of all characters, locations, and quests encountered.
- Various side-quests will be present throughout the game. They will not be important to the plot, and instead offer additional lore and unique reward items. These are usually menial tasks like bringing a plant water.
- Version 1.0.0 - This was the first release of the TS!UNDERSWAP Demo. (Released December 23, 2020.)
- Version 1.0.1 - Fixed minor bugs. (December 23, 2020.)
- Version 1.0.2 - Fixed more minor bugs and tweaked encounter rates. (Released December 24, 2020.)
- Version 1.0.3 - Fixed even more bugs, and made some minor tweaks. (Released December 26, 2020.)
- Version 1.0.4 - Fixed when people couldn't type Z key in the GameJolt login screen. (Released December 26, 2020.)
- Version 1.0.5 - Fixed GameJolt related bugs. (Released December 28, 2020.)
- Version 1.0.6 - Fixed more bugs and updated the credits. (Released December 31, 2020.)
- Version 1.0.7 - Fixed minor bugs, new quality adjustments and added official localizations for Spanish, Brazilian Portuguese and Dutch. (Released February 20, 2021.)
- Version 1.0.8 - Fixed minor issues with the official localization files (Released March 6, 2021.)
- Due to characters retaining their personalities, and their actions affecting the world around them, roles are handled very differently from traditional Undertale AUs.
- Many characters are present in areas their counterparts were not in Undertale--for instance, Bugerpant appears in Ruined Home, while Nice Cream Guy doesn't appear until Snowdin.
- The roles themselves are handled quite differently for both major and minor characters. This will result in the plot progression of TS!Underswap greatly diverging from Undertale's, with entirely new event sequences.
- Prior to its major reboot phase, each of the locations of TS!Underswap had names "opposite" of Undertale's--for instance, "Sunnedout" (now Starlight Isles) and "Snowdin", as well as "Coldsea" (now Boreal Bluffs) and "Hotland."
- An official name was never completely given, or decided on, for TS!Underswap's Waterfall replacement prior to its major reboot phase--currently, it has been named "Crystal Springs."
- Coldsea was the last of the location names to be updated, having been changed in late November 2019 for consistency reasons. The new name reflects the intended geographical features.
- Sans had been redesigned many times.
- His first major design resembles classic Underswap Sans.
- His second major design resembled his first, but had some differences--a white "X" replacing the Starman symbol, black athletic shorts with white stripes, and a torn space-themed blanket as a cape.
- Like how the Starman symbol on Papyrus's battle body in Undertale references EarthBound, the "X" on this design iteration referenced the game "OFF" by Mortis Ghost.
- His third major design was a complete overhaul, sporting a blue mask, cape, and jacket.
- His fourth and final major design is a modified iteration of the previous one--most notably, his jacket is replaced with a blue hoodie emblazoned with a white crossbones, hence his secret identity's name.
- Asgore enjoys drinking tea, tending to the flora in Ruined Home, and playing classical guitar.
- Papyrus does not smoke in Team Switched's Underswap. His original design's cigarette was replaced with a lollipop --- it was inspired by this video, beginning as a joke but then becoming part of the goofy skeleton's character.
- Papyrus's hoodie is suppose to resemble his battle body. It takes hefty inspiration from a piece of art by Tumblr user lead-namari. 
- Papyrus used to work as a scientist along with Undyne and her predecessor.
- Sans cannot change the shape of his pupils, contrary to popular interpretation. Though, he did find a pair of star-shaped glasses in the dump one time. This is a reference to the fact that the popular headcanon, cutesy version of classic Underswap Sans, Blueberry, is shown with star shaped eyes.
- Sans has the ability to use "shortcuts" like his Undertale counterpart. This allows him to perform seemingly-impossible pranks on Papyrus and his other friends.
- The name "Mettacrit" is the headcanon name given to Mettaton's ghost form in the Storyshift AU. This name is used as the canon name in Team Switched's Underswap.
- Team Switched's Underswap is perhaps the oldest of all swap takes, having been around since, at the latest, June 14th, 2016.
- While entirely speculation, it seems that TS!Underswap's transition to a more original project sparked the frequency of unique Underswap takes in the community.
- Team Switched and the creator of Inverted Fate, Dorked, are on friendly terms with one another. Team Switched has promoted Inverted Fate content (playable battle adaptations, video animations, etc.) via their social media.
- Dorked has repeatedly stated that TS!Underswap and Inverted Fate are two entirely separate projects, with neither copying from one another.
- Dorked has made a playthrough of the April Fools 2019 joke demo, Team AAA Underswap (also known as Underswaaap), on YouTube.
- "[W]e found the date the original GameJolt page was created -- February 16, 2016!" - Team Switched Twitter
- Someone named "A.S. Dreamer..." creates the comic books to accurately portray Crossbones's many heroic adventures. - Official TS!Underswap Tumblr
- Alphys is the Admiral of the Royal Navy, delivering powerful speeches to the residents of Crystal Springs. - Official TS!Underswap Tumblr
- Undyne is the Royal Scientist and owner of the royally-sponsored company, Undying Robotics. - Official TS!Underswap Tumblr
- Hey Papyrus what's Muffet like? - Official TS!Underswap Tumblr
- Gaster was the Royal Scientist before Undyne. - Official TS!Underswap Tumblr
- Syrose and Butchy! - Official TS!Underswap Soundcloud, "Tuffer Upper!" comments section.
- For anyone who isn’t aware, these are Spooky Seltzer and Woodwork, two planned original enemies for TS!Underswap! - Official TS!Underswap Tumblr
- Welcome to The Sewer! - TS!Underswap GameJolt (Halloween Update - October 2019)
- It's The City of Old! - TS!Underswap GameJolt (Progress Report - Christmas 2019)
- So, are the barriers (not the lore versions) in TS!UnderSwap able to be activated in the base resolution? - Official TS!Underswap Tumblr
- Artwork of Papyrus - lead-namari's Tumblr
- Dorked's statement on how TS!Underswap is different to Inverted Fate - Inverted Fate Tumblr
- Dorked Plays TS!Underswap (BLIND) - Dorked's Youtube Channel