Revision

A Retcon which adds to the Backstory without altering or contradicting any previously known information.

The introduction of a Cousin Oliver or Long-Lost Uncle Aesop is often a Revision, while Chuck Cunningham Syndrome is often a Rewrite. The Other Darrin may be either or both.

This often occurs in order to address a previous contradiction to continuity.

Examples of Revision include:

Comic Books

The early-1980s series All-Star Squadron ran for 70 issues, all of which was slotted in between issue #10 and #13 of the 1940s series All-Star Comics. The term "retcon" (in its longer form, "retroactive continuity") was devised by writer Roy Thomas to describe this book.
The mid-'90s comic series Untold Tales of Spider Man was designed entirely around this trope. They even included a timeline laying out where each story took place among the original Lee/Ditko stories.
- Dan Slott's Spider-Man and Human Torch and Christos Gage's X-Men and Spider-Man do similar interweaving with the timeline. It's fun to figure out how all of the retcons work around each other...
The reprint series X-Men Classic, which debuted in the X-Men's '80s heyday, often incorporated newly drawn insert panels with original dialogue by Chris Claremont to elaborate on some plot point or character note, or indeed to bring older stories in line with later plots.
X-Men: The Hidden Years
Captain America (comics)'s Super-Hero Origin was rewritten in Tales of Suspense #63 so that Steve Rogers had to drink the Super Soldier Serum instead of having it injected (due to the Comics Code Authority prohibiting demonstrations of drug use). The origin was subequently subjected to revisions -- Captain America #109 had Steve exposed to Vita-Rays after drinking the formula, and Captain America #255 reintroduced the injection of the formula while retaining the oral dosage. Grant Morrison's run on New X-Men gave the origin yet another revision—the project Steve participated in was actually a part of the Weapon Plus program, which was also responsible for the likes of Wolverine, thus retroactively making Captain America Weapon I.

Literature

The novel Death Star explains why the superlaser's firing sequence (which, when destroying Alderaan, had been activated quickly and systematically) is so slow and deliberate when it comes time to blow up the Rebel base.
The 'fourth' and 'sixth' books (chronologically) in the Anne of Green Gables series were written after the rest of the books, to appease the fanbase. They work with information that L. M. Montgomery provided in other novels.

Live Action TV

Star Trek: Enterprise, especially in its final season, was very fond of carefully challenging the common assumptions about past continuity while ensuring that everything we had heard before was still literally true (most notably, the meeting with the Borg, and the ultimate explanation of the Klingon Forehead Issue). Undoubtedly an interesting mental exercise (see Fan Wank), but quite often, the viewers are left feeling like the writers just pulled a fast one.
In The Avengers episode "The Forget-Me-Knot", Steed and Mrs Peel are revealed to have a never-before-seen superior called Mother. He (oh yes) subsequently became a recurring character for the rest of the show.
Caprica. Pretty much "Revision: The Series!". It helps that the parent show, Battlestar Galactica, revealed almost no backstory on the era of Colonial history that Caprica takes place in, giving the writers a very wide scope to tell stories without stepping on established canon.
Done spectacularly in the new Doctor Who, starting from "The Sound of Drums" and the return of the Master. The drumbeat in the Master's head goes ta-ta-ta-TA-(one-two)-ta-ta-ta-TA-(one-two)... Play that in your own mind. Remind you of anything? That's right. They applied a revision to the whole series, and one which will stay in effect until the very end.
- The writers pull it off once more in "The End of Time": it turns out that the drums are real and are being used by the Time Lords to control the Master. And the kicker? They chose that sound because it's the sound of a Time Lord's heartbeat, which adds another meaning to the title theme - it can be interpreted as the Doctor's own heartbeat.
- "The Doctor's Wife" revises the ever-vague origin story of the Doctor himself. It's widely-known canon that the Doctor wanted to see the universe, so he "borrowed" a TARDIS and ran away from Gallifrey. Neil Gaiman's revision? The TARDIS wanted to see the universe, so she stole a Time Lord (by leaving her doors unlocked for the Doctor) and ran away from Gallifrey.
- It turns out that the Doctor and friends have run into The Silence multiple times before the reveal episode - which makes sense, as The Silence are more or less retcon beings.

Newspaper Comics

Stretched to its absolute limit in the comic strip Candorville. Then again, we were never told that Roxanne wasn't a powerful Vampire trying to Take Over the World, so it was entirely possible she could be one.

Toys

In Bionicle, the storyline had a unique way of expanding both ways: as it progressed in the present, various Flash Back bits delved deeper and deeper into the past, all the while explaining or even completely rewriting present story details (or the way we perceive them) without resorting to changing the exact details themselves or contradicting facts. How much sense these revisions made, from the stories' and our own standpoint, varied greatly.

Video Games

Metal Gear Solid 4: Guns of the Patriots is able to pull this off. In MGS2, the Patriots were apparently a bunch of long-dead men who controlled America via viruses and artificial intelligences, and wanted total information control of the Earth and might have launched the ultimate Big Brother ship to do this, but were stopped. It makes a lot less sense in context. However, it's revealed the Patriots are AIs who were originally supposed to simply guide governments away from war and towards peace. However, they went berserk (though it is hinted it was more in the manner of a programming bug rather than they suddenly gaining sentience) and started causing wars and, yes, wanted total information control - the opposite of what their creator wanted. Great job on the scriptwriters for being able to subvert the Gainax Ending of MGS2. Oh, and Vamp isn't an actual vampire. While the long dead people did exist, they were just a group of really influential and rich men who had some hands in politics - not even much of a conspiracy, except the money they pooled together, which was meant to be used in case of emergency, but was stolen by one of their own.
- Done to a more subtle degree between Metal Gear and Metal Gear 2, reaching its apex in Metal Gear Solid. The events of Metal Gear were very 'video-gamey' - while there was a clever plot twist which was mindblowing at the time (a subversion of the Exposition Fairy where they were actually aiming to confuse and harass you), there wasn't anything close to Character Development. Metal Gear 2 began to give Snake some personal trauma, and by Metal Gear Solid, in order for the plot to make any sense, we must believe that the (near-plotless to our eyes) Metal Gear was, in fact, an epic war drama full of blood and terror and Heroic BSOD and Luke, I Am Your Father. It actually works very well (and is a heaven for Fix Fic writers) - until you actually replay Metal Gear. Then...
Quite common in the Warcraft universe, probably slightly outnumbering outright retcons. The most famous one would be the Orcs' shamanic heritage, though as they've continued fleshing that out details can get a little inconsistent.
Done in Portal, with a new achievement and a patch to change the game ending. The whole point is to set up Portal 2.
In Modern Warfare 3, Yuri is shown to have played a background role during several key events in the Modern Warfare timeline. He is first seen during the nuke deal held in the "One Shot, One Kill" mission from the first game (where he and Makarov are the ones who drive the injured Zakhaev to safety), as well as being present when Makarov (standing in Al-Asad's safehouse) detonates the nuke in the "Shock And Awe" mission. He is then revealed to have been present during the "No Russian" mission from the second game - Makarov shoots him in the stomach before he, Private Allen and the other Ultranationalists massacre the civilians in the airport. Yuri makes it to the lobby and witnesses the aftermath of the massacre before collapsing and being treated by paramedics.
Deus Ex: Human Revolution (being a Prequel) is an entire game full of this to the original Deus Ex.
In Mass Effect 2 its revealed that Tali'Zorah has been nurturing a crush on the Male Commander Shepard since the events of the first game. Naturally everyone onboard the Normandy already knew.
- Also a hint of it towards Fem!Shep, but it goes nowhere.

Web Comics

Done in El Goonish Shive, with the background character of the Shy Girl being merged with the minor character Rhoda.

Western Animation

Transformers: Beast Wars did this quite nicely: The revelation of Tarantulas's true loyalties in "The Agenda" is a particularly well-done example. It contradicted nothing, fit in seamlessly, and added another layer to both the plot and the character. Of course, even that wasn't the whole truth.
If the major and most of the minor retcons made within the Ben 10 multi-series don't fall under this, they eventually will. The writers have an uncanny ability to bring up plot points established earlier and reshape them to make sense. (i.e, why Kevin changed between the first and second series, how Gwen was able to adapt to magic so easily, etc.) A significant example is Ben 10: Ultimate Alien's "Moonstruck", which brings up small story elements taken from both previous series.