Richard Speight Jr. reprises his role as Loki in the series. The Winchesters is primarily related to the original show through John and Mary’s status as Sam and Dean’s parents, but there have been quite a lot of clashes within the canon.
Viewers are expected to see the unfamiliar parts of the plot as retcons, meaning they’re now the official backstory of the characters. These range from how John and Mary meet to their knowledge of the Men of Letters, along with a number of other things that have changed quite a bit compared to how they were presented in Supernatural.
Knowledge Of The Men Of Letters
The Men of Letters were so secretive in Supernatural that nobody in America knew of their destruction at Abaddon’s hands. Henry essentially took knowledge of the organization with him to the future when he escaped to 2013, with the Men of Letters becoming defunct.
However, The Winchesters’ pilot episode showed John and Mary coming across a Men of Letters bunker that they’ve used as their base of operations. Moreover, the characters have become aware of the Men of Letters organization, which retcons Sam and Dean learning about them decades later in Supernatural.
John's Knowledge Of Mary's Status As A Hunter
While it was made to seem that Dean shared John’s personality traits in Supernatural as a hunter, The Winchesters has shown him to be a lot more like Mary was in her youth. Of course, the fact that John even knows about Mary’s status as a hunter is a huge retcon.
Supernatural featured an episode where Dean traveled to the past and learned John was just a civilian who had no idea of Mary’s true background. The Winchesters has changed this to avoid making the characters’ entire relationship based around Mary’s secret to go with the latter teaching John how to be a hunter.
The Ages Of The Characters
Quite a bit of John’s personality goes against how he was in Supernatural, but The Winchesters also appears to have aged him up. Supernatural had revealed both John and Mary to be born in 1954, which would make them 17-18 in The Winchesters.
However, the prequel has changed their ages to 19-20 – the show explained that John lied about his age to the Vietnam War, but he still couldn’t have been 15 years old as his Supernatural age would imply if it's aligned with The Winchesters' timeline.
John Having Any Allies In The Hunting World Before Mary's Death
What set apart Supernatural from other horror shows like Buffy was that there wasn’t a gang of characters who fought monsters. The few allies Sam and Dean had as youths were due to John’s connections, whom he was said to have met after he became a hunter when Mary died.
The Winchesters has retconned this in quite a big way by showing that John and Mary had a team of their own. Carlos, Latika, and Ada are part of the original roster of allies that John and Mary have, with new episodes expanding their group of friends – none of these were mentioned to exist in Supernatural.
The Akrida's Status As A Bigger Threat Than The Apocalypse
There have been many great villains in the history of Supernatural, with the likes of Lucifer and Michael considered the biggest since they threatened the world. Their fated clash was the reason Sam and Dean were born, and the apocalypse was averted by the heroes to prevent them from destroying the planet.
However, The Winchesters has retconned this in favor of the Akrida, who are supposed to be so much of a threat that they are an issue for all existence. This makes them a bigger deal than the apocalypse since the latter only threatened Earth, while the Akrida seems to go beyond time as Dean is researching them in the future.
Characters' In-Depth Knowledge Of Demons
Demons were a huge deal in the first season of Supernatural, to the point that encountering one terrified the heroes. This meant that there wasn’t much knowledge about these enemies in the past, only for The Winchesters to reveal that hunters know quite a bit about demons.
This was shown when Carlos effortlessly exorcised a demon in the first episode, along with Ada trapping a demon inside a tree through a different form of exorcism. Within the series, it’s now made to seem as if Sam and Dean specifically just didn’t know about these techniques.
John Learning That His Father Didn't Abandon Him
Supernatural implied that John was bitter about his father’s disappearance throughout his life and believed that Henry had walked out on him. The Winchesters started along the same lines before John found out about Henry’s status as one of the Men of Letters.
The prequel has retconned John’s issues with Henry’s disappearance from his feelings of abandonment to wanting answers over why his father went missing. The fact that John knew that Henry didn’t willingly leave him behind changes the context of Sam and Dean eventually meeting their grandfather in the future.
The Way John And Mary Fell In Love
The biggest retcon easily has to be the circumstances behind John and Mary’s romance, which had been said to be the work of a cupid in Supernatural. The original show had a Cupid claiming that the two couldn’t even stand each other when they first met before he worked his magic on them.
The Winchesters has changed this to show that John and Mary had an instant connection when they first met, with neither character showing any hostility toward the other. Furthermore, John and Mary’s relationship will blossom from friendship into a romance based on their experiences hunting together, which Supernatural never hinted at.