Retcon

Retroactive continuity, or retcon for short, is a new piece of information that imposes a different interpretation on previously described events. In regards to the, a retcon is a situation in which the storyline of a new game explains or changes an event from a previous one, or attaches a new significance to it.

Retcons are typically used in the Metal Gear series to facilitate a dramatic plot shift or account for an inconsistency. Some retcons add information that contradicts the information that was previously established, while others instead fill in missing background details, allowing the accommodation of prequels/sequels or acknowledgement of real world developments. While creating the series, Hideo Kojima was willing to contradict established continuity on occasion, in order to ensure that "new games are as good as they can be."

Metal Gear 2: Solid Snake
Retcons in Metal Gear 2: Solid Snake (MG2) mainly expands upon events that occured in its predecessor Metal Gear (MG). Some information is given outside the main narrative, i.e. the user's manual or optional radio conversations.

Metal Gear Solid
The most notable retcon in Metal Gear Solid (MGS1) concerns the resurrection of the presumed-dead Gray Fox from Metal Gear 2, along with the previously unseen relationship between Solid Snake and Big Boss. Background details given in the "Previous Operations" story recaps (Prev. Ops) and user's manual, alter pre-existing information given in the manual for Metal Gear 2, mainly to reflect real world events regarding the end of the Cold War, but also to alter or expand the histories of Snake, Big Boss and FOXHOUND.

Metal Gear Solid 2: Sons of Liberty
The retcons in Metal Gear Solid 2: Sons of Liberty (MGS2) are largely contributed by the supplementary in-universe novelization In the Darkness of Shadow Moses: The Unofficial Truth (MGS2 Dark).

Metal Gear Solid 3: Snake Eater
As the first prequel in the series, Metal Gear Solid 3: Snake Eater (MGS3) introduces many retcons concerning its protagonist Naked Snake (Big Boss) and the origins of Metal Gear.

Metal Gear and Metal Gear 2 re-releases
The re-released versions of Metal Gear and Metal Gear 2 changes the names of several characters in the games. In addition, all the character portraits were given new and/or updated designs by Yoji Shinkawa in the latter to replace the dubiously legal use of digitized photographs of famous figures.

Metal Gear Solid: Portable Ops
The spin-off game Metal Gear Solid: Portable Ops (MPO) expands upon some of the events of its predecessor, Metal Gear Solid 3. It also introduces many changes to the backstory of Frank Jaeger, a.k.a. Gray Fox, contradicting previous accounts from Metal Gear 2.

Although MPO is no longer considered a part of Kojima's core "Metal Gear Saga", its story serves as a prelude to Metal Gear Solid 4: Guns of the Patriots, where some plot points are revisited.

Metal Gear Solid 4: Guns of the Patriots
As the intended grand finale of the series, Metal Gear Solid 4 (MGS4) features extensive retcons in its attempt to wrap up the remaining plot threads from all previous games, some of which had deliberately been left open for interpreation.

On a technical level, a large number of changes are made to the layout and contents of the Shadow Moses Island facility, last seen in the original Metal Gear Solid (see Shadow Moses Island for more details). For example, there is an entire casting facility to the north of the blast furnace area, accessed via an elevator that was not present in the earlier game.

Metal Gear Solid: Peace Walker
Metal Gear Solid: Peace Walker (PW) alters or expands upon the backstories of several returning characters.

Metal Gear Solid V
Like other prequels, Metal Gear Solid V: Ground Zeroes (GZ) and Metal Gear Solid V: The Phantom Pain (TPP) expand on the backstories of returning characters.