Superman's origin, early years, and powers have been revamped a ridiculous number of times just in "official" comic book continuity (and not counting in-story changes). Probably the most notable and drastic example took place in John Byrne's Man of Steel, commissioned by DC in the 1980s to "clean up" the past several decades of Superman continuity by revamping his origin and the story of how he began his superhero career. Among other things, this retcon scaled back Superman's powers from the levels they had been inflated to (although they have since begun to creep back up a bit), re-established Superman as the only surviving Kryptonian (that one didn't stick either), and wiped out previous continuity in which Clark Kent had a hero career as a teenager in Smallville using the name Superboy, during which time he also befriended the young Lex Luthor in How Luthor Met Superboy.
Byrne retconning Superboy's career out of existence completely borked the continuity of the Legion of Super-Heroes comic, since the eponymous Legion was introduced in a 1958 ''Superboy'' story as a group of thirtieth-century teenagers who were inspired to form their own "hero club" by stories of Superboy's exploits. The Legion's writers at the time tried to patch things up by, variously, establishing that Superboy had only existed in a pocket universe, killing off the pocket universe Superboy, revamping one-shot character Mon-El into a Superboy Expy, further rejiggering the timeline by having Mon-El kill the Time Trapper, and finally scrapping and rebooting the whole damn thing during the Zero Hour: Crisis in Time! crossover in 1994. The Legion of Super-Heroes was then rebooted again in 2005, and then retconned again in 2007 back to a variant on the original continuity, with some adjustments. By this point, alternate timelines, retcons, and reboots are a fact of life for Legion fans.
In the Legion's first story, several unidentified Legionnaires show up in the team's headquarters. Reprints and trade collections will retcon one of them into being Brainiac 5 -who had joined years later in - the Supergirl's Three Super Girl-Friends story- by recoloring his hair and head blonde and green.
Legionnaire Lightning Lad got his powers by being struck by the lightning of some monsters. Then it's revealed he has an older brother who was there as well! Then it's revealed in The Death of Lightning Lad he has a twin sister, who was also there.
Originally, Superman's powers were inherently genetic because he was from a race of "supermen" — this can be found in the prologue of several Fleischer shorts. Eventually, it was rewritten that his powers came from Earth's yellow sun. Additionally, Superman's strength and invulnerability were attributed to being super-dense. Post-Crisis, his powers came directly from the yellow sunlight. When Superman met the "pocket Superboy", the latter wiped the mat with the adult Supes, because he had Pre-Crisis power.
Pre-Crisis, Superman's invulnerability and powers manifested immediately, leading to the adventures of "Superbaby". Post-Crisis, it took 18 years for Clark to develop his superpowers fully.
Another major Superman retcon that most people don't know about is his attitude. Siegel and Shuster originally wrote him as very rough and aggressive. On one occasion he kidnapped a slumlord, trapped the man in one of his own shoddy buildings, and threatened to collapse the whole structure on top of the guy if he didn't promise to improve conditions for his tenants. He also smashed a wife beater into a wall. A far cry from the Big Blue Boy Scout we all know and love today. World War II shifted his priorities into patriotism and he became a champion for "Truth, Justice and the American Way". When the Comics Code Authority came into being in the early 50s, its restrictions on characters' behavior ensured Superman became really square.
Also happens to Superman's cousin, Supergirl. Not only have there been four separate versions, but the pre-New 52's version's history became so convoluted on its own that Sterling Gates just retconned it out in issue 35, to give her the simple story we all thought was true before. The reason for the massive continuity snarl around Supergirl is because of an editorial mandate that Superman be the only surviving Kryptonian when his own continuity was rewritten and simplified after Crisis on Infinite Earths. This caused obvious problems for Supergirl (which in turn broke the Legion of Super-Heroes, among other things, since it was heavily interconnected with her), and necessitated increasingly convoluted explanations until they finally just threw up their hands, admitted that casual readers would always assume she was Superman's surviving cousin regardless, and switched it back to that in The Supergirl from Krypton (2004).
Superman Reborn: Superman's origin and past exploits all mostly happened, thanks to the pre-Crisis and New 52 Superman and Lois Lanes merging. The events of Superman: Secret Origin, his 80s and 90s adventures, his marriage to Lois Lane and other things all happened. The New 52 adventures happened while Lois was pregnant with Jon Kent and Jon was born in the Fortress of Solitude, not the Flashpoint Batcave as established in Convergence. The events of Superman: Lois and Clark are explained as part of a sabbatical where Superman focused more on being a father than a hero, allowing Lex Luthor to become his own Superman.
Batman/Superman: World's Finest restores a pre-Crisis on Infinite Earths element as it restores a version of Supergirl's original debut, The Supergirl From Krypton (1959), including Argo City surviving the original destruction of Krypton as a domed city, only for its inhabitants to die due to the ground turning into Kryptonite and a meteor shower destroying the protective dome and led shielding. forcing her father to send Kara to Earth, where she's found by Superman and briefly put into an orphanage. About the only thing kept from Kara's pre-Post-Crisis origins was her being born on Krypton before its destruction and chronologically (but not physically or mentally) being older than Clark.
