Metamorphosis

A permanent, one-way case of Shapeshifting. As such, it's generally taken a lot more seriously by the subject, and less likely to actually happen. Voluntary metamorphosis is generally only agreed to after much thought; involuntary metamorphosis provokes much angst and anger at the responsible party.

Malicious examples can be inflicted as a form of Cool and Unusual Punishment. Benign examples can allow a character to Ascend to A Higher Plane of Existence. May overlap with Karmic Transformation, Pinocchio Syndrome, or Baleful Polymorph.

Compare with Shapeshifter Mode Lock and First Law of Gender Bending. Not to be confused with the usual Transformation Sequence, despite both Tokyo Mew Mew and Yes Pretty Cure 5 using "Metamorphose!" as their trigger phrase, and "morph" being a key word in Power Rangers.

Advertising

 * A series of commercials for the Ratchet and Clank series showed weapons from the games being used in real life in a Jackass inspired way. Many of these included involuntary one-way transformations, where one guy gets turned into a chicken despite protesting, a few friends turn one of their mum's into a sheep by accident and two guys transform one of their friend's girlfriend into a cow just to mock him.

Anime and Manga

 * Although the use of "Metamorphose" is unrelated, Yes Pretty Cure 5 did this with most of the Quirky Miniboss Squad in their final appearances, turning them (usually against their wills) into giant superpowered versions of themselves at the cost of apparently all their higher brain functions. The Big Bad and The Dragon were similar, but dodged that last part.
 * Hideshi Hino's The Bug Boy is the story of a kid who is reborn as a giant bug. He suffers pretty much the same fate as Kafka's (below) protagonist.
 * Franken Fran regularly features plots like this, although the protagonist's super-surgery, the transformations usually to turn out to be reversible (and into something much worse...)
 * Metamorphosis is a recurring theme in This Ugly Yet Beautiful World. The first half of the series is all about Fanservice and Slice of Life meets supernatural weirdness. Then Cerebus Syndrome and rampant butterfly symbolism kick in.

Comic Books

 * Comet the Super-Horse, from pre-Crisis DC, was originally a centaur called Biron who wanted to be fully human. Unfortunately Circe made a mistake and made him fully horse instead. Because the spell couldn't be reversed, Circe gave him superpowers to try and make up for it.
 * The Teenage Mutant Ninja Turtles are all up in this trope. Aside from the turtles and Splinter themselves, there's Leatherhead, Bebop and Rocksteady, Baxter Stockman (in the first cartoon), the Mighty Mutanimals in the Archie comics, etc.
 * In Megalex, Zerain's hump bursts and a pair of wings sprout from his back when he and Ram win the battle of the twelve chiefs. This is a sign that he's The Chosen One.

Fan Works

 * John is permanently changed into a Winged Humanoid (with various attendant perks and problems) in With Strings Attached. It's done to save his life by the Fans.

Film

 * The entire premise of The Fly.
 * In Disney's version of Aladdin, Jafar is transformed into a genie.
 * On the other hand, *being* an all-powerful genie, in the sequel he gets to appear as human as he ever was whenever he feels like it.
 * The movie Wolf combines this with Our Werewolves Are Different.
 * The transformation into a werewolf in Ginger Snaps is pretty much this.
 * District 9.

Literature

 * Pinocchio is a wooden puppet who wants to be a real boy. Eventually he gets his wish.
 * In Roald Dahl's The Witches, the witches plan to dispose of all children by turning them into mice.
 * In Franz Kafka's appropriately named story The Metamorphosis, the protagonist Gregor Samsa finds himself turned into a giant cockroach insect at the very beginning.
 * Evil Magician Trent from Xanth has the power to turn any living creature into any other. He sometimes uses it on people who annoy him, like Cynthia Centaur who was formerly Cynthia Human.
 * In the little-known 1980's SF/Horror novel Transformation by Edmund Plante the pregnant heroine is horrified to discover that an alien virus is transforming her into a psychic slug-and-lizard hybrid... at least until she she decides that a pregnant psychic alien slug/lizard is actually a pretty cool thing to be.
 * In Jack Chalker's better-known 1970's novel The Web of The Chozen a human hero is transformed by an alien virus into an alien creature. One attempt at getting help from his superiors is enough to convince him to abandon humanity in favor of his new species.
 * The twist behind The Soddit is that the Galdalf Expy is transforming into a dragon- as part of his natural life cycle: dragons begin life as dwarfs, grow into wizards, and finally become dragons.
 * Some Children Wander By Mistake by John Connolly features a child being permanently transformed into a monstrous clown, having been chosen as a fresh recruit by the other clowns several months before he was born.
 * The Ray Bradbury short story "chrysalis" details the metamorphosis of a man exposed to radiation. Other characters include a co-worker of his and a couple scientists studying him as he transforms. The entire story takes place while he is in the cocoon (or chrysalis), and everyone wonders what to do with him, if he's dead, etc. Things are further complicated when it's revealed his co-worker is showing similar symptoms and likely the same will happen to him. In the end, the man emerges from his chrysalis... and is perfectly normal.
 * The premise for Edgar Rice Burroughs' "The Land That Time Forgot" is that all the creatures on the island are evolving from one species to another as they move inland and northward.

Live Action TV

 * In Buffy the Vampire Slayer, a witch's rat-transformation spell turns out to be permanent when she uses it on herself - or at least, she can't undo it while transformed.
 * She got better.
 * In Babylon Five Delenn enters a cocoon in order to turn herself into a human/Minbari hybrid.
 * Sinclair goes through the same process in reverse to become Valen.
 * In Deep Space Nine, Odo was turned into a solid (i.e. a non-shapeshifter) as a punishment for opposing his species' plan to take over the galaxy.

Mythology

 * Numerous Greek myths involved permanent transformations. Ovid's Metamorphoses (the Trope Namer) featured a number of such stories.
 * The nymph Scylla was desired by the merman Glaucus (although she brushed off his advances). The sorceress Circe also loved Glaucus, and in jealousy poisoned Scylla, transforming her into a twelve-footed, six-headed monstrosity. Scylla adjusted well to the change: she moved to a rock on the strait opposite Charybdis (also a transformed nymph) and took to devouring sailors from every ship that passed beneath her.
 * Arachne and the goddess Athena had a weaving duel. There are many versions of this myth, and the only detail they can agree on is that the duel ends with Athena turning Arachne into a spider.
 * Actaeon, prince of Thebes and renowned hunter, was transformed into a stag by Artemis and killed by his own hounds. Depending on the version, this was either punishment for spying on Artemis bathing, or for boasting that he was a better hunter than Artemis.
 * Similarly, when Siprotes saw the goddess bathing, he got changed into a woman instead. The difference, though, was that Actaeon had gazed at Artemis, while Siprotes tried to avoid eye contact as soon as he saw her.

Tabletop Games

 * Medeans voluntarily undergo a transformation to become Medeans.
 * The Elans of Dungeons and Dragons are humans who have undergone a secret, voluntary transformation that leaves them looking exactly the same, but they are effectively immortal, being able to live purely on psychic energy. There is just something off-putting about them that makes the normals uncomfortable, though: by game rules, they have become Aberrations like a rust monster or a mind flayer.

Toys

 * In Bionicle,

Video Games

 * In the modern usages of both words, "metamorphosis" would be a much more accurate term than "evolution" for when this happens in Pokémon. The reason it was called evolution probably has something to do with "metamorphosing" being huge and unwieldy (and the shorter "morphing" being permanently associated with Power Rangers).
 * The latter character from platforming series Jak & Daxter for coming into contact with dark eco.
 * In the Rance Series, the Whale God turned the Dragon King into an anthromorphic cat for breaking the Balance Between Good and Evil. Note that the Dragons were on the side of good and won. That's how much of a Jerkass God Ludo-Rathowm is.

Webcomics

 * In Gunnerkrigg Court, metamorphosis accompanies immigration and emigration to and from Gillitie Wood. Regional Fairies and other forest-folk become humans in order to leave Gillitie and enroll at the Court, and a family of humans transform into birds before moving into the Wood.
 * In Exiern dragons are born looking human and don't metamorphose into their true form until they reach adulthood. Adult dragons destroy human settlements and plant their young amongst the suvivors to be raised by humans like cuckoos. The heroine's pretty sure her romantic interest is a dragon (she barely survived an encounter with his half-transformed sister) but hasn't figured out how to tell him.
 * Tony of Skin Deep turns into some kind of bird creature after a spell keeping him human breaks (we think)

Truth In Television

 * Metamorphosis is a natural process for various species of animals, where their juvenile forms look little (or nothing) like the adult. The transformation of tadpoles into frogs and caterpillars into butterflies are two of the best-known examples, with butterfly metamorphosis transcending to the point of metaphor.