"Retcon” is a tradecraft term in serial commercial fiction. From Wikipedia:

Retroactive continuity, or retcon for short,^[1][2] is the alteration of previously established facts in the continuity of a fictional work.^[3]

There are various motivations for retconning. The changes may occur to accommodate sequels or derivative works, allowing newer authors or creators to revise the diegetic (in-story) history to include a course of events that would not have been possible in the story's original continuity. Retcons allow for authors to reintroduce popular characters and resolve errors in chronology.

Retcons are common in pulp fiction, especially in comic books published by long-established publishers such as DC and Marvel. The long history of popular titles and the number of writers who contribute stories can often create situations that demand clarification or revision. Retcons also appear in manga, soap operas, serial dramas, movie sequels, professional wrestling angles, video games, radio series, and other forms of serial fiction.

Science fiction writers are occasionally confronted with new scientific developments which disprove assumptions made in a previous story or book. For some of these cases, no amount of retconning could "save" a story – for example, many early works of science fiction assumed that Venus was a watery planet – but in many other cases, clever retconning allows the story to retain scientific plausibility under the new conditions.

But of course retconning is also done for ideological purposes, to blame Bad Guys for disasters. Winston Smith in 1984 retcons old newspapers for the Ministry of Truth.

One of the most spectacular jobs of retconning of my lifetime was the rewriting of the history of the AIDS epidemic in the United States. Obviously, the chief engine was Gay Liberation (with heroin number two). The epidemic was most intense not just in cities where Gay Lib triumphed first, but on gay streets: Castro, Christopher, and Santa Monica.

But that’s a little too much nuance. Gays are good, Gay Liberation is, by definition, great, so AIDS must have been caused by Emmanuel Goldstein. So, the national press has retconned recent history to make the AIDS Bad Guy … Ronald Reagan!

From Slate:

Hugger-in-Chief

In embracing an Ebola victim, Barack Obama succeeded where Ronald Reagan failed.

By Laura Helmuth

Slate’s Science and Health Editor

… The hateful, homophobic, racist response to the AIDS crisis is one of the most shameful episodes in recent American history. Within a few years after the first AIDS cases were reported in 1981, scientists knew the disease was transmitted primarily by sex, blood transfusions, and shared needles.

That knowledge didn’t stop the prejudice and fear mongering. HIV-positive people were fired from their jobs, forbidden from entering the country, kicked out of the military. Jerry Falwell claimed AIDS was “God’s punishment not just for homosexuals” but for a “society that tolerates homosexuals.” William F. Buckley Jr. wrote in a widely syndicated column that people diagnosed with AIDS should have that fact tattooed on their buttocks. Schools refused to enroll children with HIV. …

Reagan could have spoken out against panic and called for compassion; the man knew how to give a powerful speech. He could have hugged an AIDS patient, or at least shaken hands.

Of course, widespread fears among the public were promoted by respectable spokespeople like Oprah and Magic Johnson who claimed that HIV didn’t discriminate, that everybody was at risk. In reality, the one broad public health danger was that the blood supply for transfusions was infected, which was pretty awful, and killed a bunch of hemophiliacs and surgery patients.

Fortunately, a test was developed fairly quickly and policies for discriminating against male homosexuals and needle junkies were quickly put in place at blood banks. Otherwise, outside of junkies and male homosexuals who liked to catch as well as pitch, Americans were in little danger of catching HIV. But that didn’t stop the respectable press from stoking sleepless nights, and then retconning history to pin the blame on Reagan of all people.