In Battlestar Galactica (2003), Sam Anders is hit in the back of the head by a bullet during Gaeta's mutiny, and left brain-dead after it's removed. As he's a humanoid Cylon, the other Cylons hook him up to a tank in the same manner as a Hybrid — the living CPU of a Cylon Baseship — in an attempt to reboot him, which has partial success, and he essentially becomes the Battlestar Galactica's Hybrid.
Mitchell is faced with this twice in Being Human (UK). In an early episode, he is confronted with a dying colleague in an alleyway. George begs him to save her. He does nothing. In the next scene, when George is asked "Why didn't he save her?", he replies "I think he did." In a later ep, he does this for an injured boy.
In the distant Back Story, the Master did this for Darla as she lay dying of syphilis. Then on Angel, a human-again but still dying Darla tries to get Angel and other vampires to turn her again. Angel refuses, but Darla is ultimately turned back into a vampire by Drusilla — only after Angel had convinced her not to want that.
Later but still in the Back Story, Angel is stuck on a submarine and the only American crewman who knows how to repair it is fatally stabbed by an escaped Nazi prisoner, so Angel reluctantly makes him into a vampire. This is the only time Angel has turned someone since regaining his soul, and the effect is that the American officer, who was young and idealistic, inherits just enough of his soul that he feels hollow doing all the typical evil vampire things. He comes back in the modern day asking Angel to give him a purpose in life, but Angel stakes him.
Also, Cordelia is turned into a part-demon by Skip to avoid her being killed by her visions. However, other than some glowy effects and cool powers like levitating and cleansing evil demonic infestations, this doesn't actually manifest itself too much, and she becomes a "higher being" soon afterwards, which turns out to be a trick to allow Jasmine to come to Earth in her body.
A variation on this happens to Illyria: the gang needs to remove some of her power before she goes supernova and destroys the whole world and herself. She is, of course, none too pleased at first when the procedure works, because "[she] would rather be a titanic crater than be like unto you", but she comes to terms with it.
Also, in the Buffy episode "Lie to Me", Ford, a childhood friend of Buffy's, tries to get Spike to turn him before he dies of a brain tumor, in exchange for luring Buffy into a trap and offering a whole bunch of deluded Vampire Vannabes as a side dish. Spike does so, since Ford did hold up his end of the deal despite Buffy getting the better of Spike after being delivered to him, but Ford is immediately staked by Buffy upon rising from his grave.
Averted in the Season 7 episode "Get It Done", in which the Shadow Men attempt to give Buffy the strength to defeat the First Evil by infusing her with the essence of a demon, in the same way they created the original Slayer. Buffy emphatically refuses with a What the Hell, Hero?, and then soundly kicks all three of their asses.
In Chuck, Sarah uploads the Intersect 2.0 when she's trapped in a firefight. In the series finale, Chuck himself re-uploads himself with the Intersect 3.0 in order to disarm a bomb.
Time Lords' regeneration abilities kick in automatically when they're about to die, though they don't get any extra powers by regenerating. At least, not in the televised series (though they do have a lot of energy flowing through them in the early stages, according to "The Christmas Invasion" and "Let's Kill Hitler"); there's a reference in one of the Eighth Doctor Adventures books to Time Lord soldiers who had "force-regenerated themselves until their skins had been covered in black organic blast-proofing". And unless granted additional regenerations by the High Council, each Time Lord can only regenerate twelve times, making a total of thirteen incarnations.
The Eighth Doctor Adventures two-part story Interference by Lawrence Miles featured a Gallifreyan priest who had been given an extremely early version of regeneration tech. As a result, his future lives included a shapeshifter, a cyborg, a heavily scarred man, a freak with a very large head, a serpent-woman, and a little grey thingy that was apparently wired directly into Time. His thirteenth life was basically an Eldritch Abominationthat became a planet's entire ecosphere.
Another angle is given in "Dalek", in which the namesake wants to die rather than live with human morality.
"The Empty Child"/"The Doctor Dances": The alien nanites rebuild Jamie but, not having a template, believe humans have gas mask faces. Once they reprogram themselves after scanning Nancy, they repair Jamie and the rest of the gas mask-faced patients.
"Rise of the Cybermen"/"The Age of Steel": When Mr. Crane destroys Lumic's life support system, the Cybermen convert Lumic into the Cyber-Leader to save him, ignoring Lumic's demand that they wait till they run out of other options.
"Last of the Time Lords": The Saxon Master subverts and averts by choosing death over regenerating and being a prisoner. Of course, he had a plan to get better in a different way, it turns out.
The Backstory of "Silence in the Library"/"Forest of the Dead" involves this trope crossed with Brain Uploading: Namely, the Library's computer is actually the mind of a terminally-ill girl whose rich family built the place and uploaded her so she could live forever.
The Twelfth Doctor does this to his companion Clara in "Hell Bent". After Clara is Killed Off for Real, the Doctor uses time travel-related tech to extract Clara out of time the moment before her death. She must someday return to this moment to prevent a universe-destroying paradox, but until that day, her life processes (breathing, heartbeat, aging) are frozen and she is functionally immortal and can put her date with death off for as long as she feels like. He intends to erase her memory of him in the hopes this will prevent the paradox and allow her to live a normal life without returning to the moment of her death, but she rejects this and erases his memory of her instead, going off with Ashildr in a TARDIS of their own for an indefinite amount of time before she ultimately chooses to die.
After Juliette's debut is attacked, Sebastian is dying because he's the only vampire present who isn't a full legacy. Davina is convinced to save him, which she does by placing the Queen Serpent within him and making him a full legacy.
Juliette and Cal find Theo bleeding out and practically dead. It's only when Cal has left that Juliette hears them ask for help and bites him. She claims she was trying to drain Theo to ease his suffering, but it's implied she knew entirely what she was doing and intended for him to be turned.
Nick acceded to Natalie's request to vamp her brother when he was fatally shot. It didn't go well.
Then in the final episode, Nick's human partner Tracy gets fatally injured, and a guilt-ridden Nick is about to bring her across when Natalie interrupts. Natalie had earlier proposed using the method that worked on Janette to make him human, but as it could have killed Natalie or turned her into a vampire he refused. So she's not impressed that Nick was about to turn someone else without even knowing if they wanted it.
Serena the Golden Hind from Hercules: The Legendary Journeysdies a Dropped a Bridge on Him sorta death, but a Time Travel story later sees her return, and near the end, she's wounded but is turned into a healthy human by Ares... still sad since she was pretty awesome as herself, and was the last of her kind, and her relationship with Herc was undone, but at least she survives.
Ice Fantasy: For the first few episodes Ying Kong Shi is the equivalent of about 14 years old and magically completely unskilled. Then he gets captured by the Fire Tribe, transforms into an adult who suddenly knows all sorts of spells (he's confused himself), easily frees himself, and stays like that for the rest of the series.
In the Showa era, this was a fairly common reason for the main character to gain their superpowers. Kamen Rider V3, Kamen Rider X, and Kamen Rider: Skyrider were all originally converted into cyborgs after suffering serious injuries.
Kamen Rider 555: The Arch Orphnoch can cure other Orphnochs of the cellular degradation that ensures they'll die within a few months to years of gaining their superpowers, but at the cost of permanently locking them into their monster forms.
Kamen Rider Blade: The title character performing one of these ends up being the resolution of the plot: in order to keep the Battle Fight from ending with a Joker as the only Undead left standing, which would bring about the end of the world, Blade deliberately overuses his Super Mode until it turns him into a second Joker.
Kamen Rider OOO: Ankh, a severed arm in need of a body, latches onto a fatally injured policeman to be his new body in the first episode. Both Ankh and his host do eventually recover enough that they're able to separate.
Kamen Rider Gaim: After a wound infected by Alien Kudzu gives him perhaps weeks left to live, Kaito opts near the end of the series to instead use it to transform into an Overlord, giving himself the near-godlike powers he needs to claim the actually godlike powers that are the only thing capable of saving his life. The transformation itself cuts his remaining lifespan from weeks to hours.
Kamen Rider Drive: Drive's Transformation Trinket is sentient due to containing the mind of its creator, who transferred himself into the belt after being attacked by a group of killer robots.
Luke Cage (2016): The experiment that gave Luke his super-durable skin was the result of an attempt to save his life after Shades and Comanche beat him into the prison infirmary for planning to snitch on Rackham.
Subverted on Moonlight, of all shows. Josh is shot, and Beth begs Mick to turn him into a vampire, but Mick refuses, and Josh dies. This weighs on Mick and Beth's relationship for the rest of the series.
Now and Again: Michael Wiseman is killed in a subway accident but secretly given the chance to have his brain placed inside a genetically bio-engineered body with superhero-like abilities.
The Outer Limits (1995): The episode "Music of the Spheres" has aliens subjecting the whole of humanity to signals that people think are music, but causes mysterious changes. Instead of the Cruel Twist Ending the series is known for, it turns out it's an Emergency Transformation into bald, large-headed, golden-skinned creatures, so that they can survive an impending shift in the sun's radiation. The aliens' process initially only works on people close to puberty, but once the humans figure out exactly what the hell is going on and why, they manage to enhance the process so that it can be applied to anyone. Some of the characters refuse to go along with the transformation; as one of the main characters puts it, he wants his wife to be able to recognize him in Heaven.
In Power Rangers Time Force, this is revealed to be the origin of Frax, Ransik's right-hand 'bot. Frax had been Dr. Louis Fericks, a scientist who had temporarily cured Ransik of a poison that was slowly and painfully killing him by administering a serum he had created. Ransik thanked Dr. Fericks for the serum by blowing up his laboratory, seemingly killing him. Fericks survived (barely), and rebuilt himself as a robot, then began his plan to get revenge on Ransik. Meanwhile, Mesogog's right-hand man Zeltrax was saved by Mesogog himself in this manner.
Stargate SG-1 (of course) had instances of this, when characters stricken with fatal injuries or diseases had the option to become a host to a Tok'ra symbiote. In one notable occurrence, the character in question was dead set against the idea, but when the illness progressed far enough, they went ahead with it anyway. There was also the option of ascending to a higher plane of existence, but not everyone could do that.
In the original Star Trek: The Original Series pilot, "The Cage", the Talosians save Vina's life but not having a template for human anatomy left her disfigured.
In the episode "Life Support", Vedek Bareil is seriously wounded in an assassination attempt while concluding negotiations for a permanent peace treaty with Cardassia. Dr. Bashir keeps him alive using increasingly extreme measures while he talks Kai Winn through the finalities, ultimately replacing a large portion of his brain with cybernetic implants. The operation has the effect of removing most of his personality, and he only survives long enough to complete the negotiations.
This was part of Dax's The Nth Doctor in the last season. Ezri Tigan was the only un-joined Trill available at the time, so they did an emergency implantation to save the symbiont and created Ezri Dax.
This trope is played with this when, in the first episode of season 9, Sam is dying and Dean makes the decision to let an angel use his body as a vessel in order to save his life from the inside.
Also in season nine, a currently human Castiel is captured by a faction of angels who torture him for information he doesn't have and refuse to believe/don't care that he doesn't have it. When his torturer's boss leaves them alone, however, the torturer offers to help him if Castiel will contact Metatron and convince him to let the torturer join his faction. This is one of the things Castiel already told them he can't do, since Metatron simply used him and they are not even remotely friends at this point, but since the torturer won't believe that, Castiel pretends to agree and is temporarily released to do it. He quickly grabs the nearest angel blade and steals his torturer's Grace before killing him, transforming back into an angel to heal his injuries and have the power to fight his way out of the compound. In a variation of the trope, he was originally an angel to begin with, but he's not particularly happy about the brutality of what he had to do to survive, and the stolen Grace in fact becomes an issue later, as it's not fully compatible with him and he suffers physically from the fall out.
In another episode, a werewolf's sister is dying from a car accident, so the werewolf bites her, assuming (correctly) that becoming a werewolf would heal the injuries and save her sister's life. The werewolf in question is also a good person who subsists on animal hearts, so she teaches her sister to do the same. This does not end well.
A variant in Teen Wolf: Scott is trying to save Liam from falling off a building, but his hands are forcibly restrained. As Liam is about to fall to his death, Scott is forced to use his teeth, thus the transformation was an unavoidable consequence of averting Liam's death rather than a direct means of preventing it.
In the Japanese series Ultraman, the first of the Ultraheroes (and some of the later ones) was created when an alien bonded with a human to save his life.
In the season two finale of Wizards vs. Aliens, Lexi pulls a Heroic Sacrifice to save Tom and winds up dying. Tom, meanwhile, consumes a huge amount of magic and gains godlike powers. When she comments that not even his power can cure what's destroying her body, he replies "Then have a new body" and makes her human, sending her away to live with their alternate universe/timeline son, Benny Jr..
