The Mandalorian retcons Jango Fett's origin story. The character was introduced in Star Wars: Episode II - Attack of the Clones as a bounty hunter Obi-Wan Kenobi was tracking as he investigated the attempted assassination of Padmé Amidala. It's revealed in the film Jango was the basis for the Republic's clone army, which was commissioned in secret. As part of his agreement with the Kamino cloners, Jango requested an unaltered clone for himself, raising the boy as his son, Boba. After Jango's death, Boba would follow in his father's footsteps as a feared bounty hunter.
Boba Fett was one of many established Star Wars characters The Mandalorian season 2 incorporated, initially teasing him in season premiere "The Marshal" before giving him a larger role in "The Tragedy." The episode was noteworthy for many reasons, including finally giving Boba a chance to shine onscreen in an action-packed set piece. It also provided additional insight on the Fett family's history, altering preconceived notions of Jango's relationship with the Mandalorian people.
Towards the end of "The Tragedy," Boba shows Din Djarin a chain code that's been encoded in his armor for more than two decades. From this, Din realizes that Jango was a foundling, just as he was at one time. Boba also says Jango fought in the Mandalorian Civil Wars. While Din was initially hesitant to give Boba the Mandalorian armor, the chain code revelation lets Din know the Fetts are the rightful owners of the armor. It's an interesting detail that retcons how Star Wars canon previously treated Jango.
In a Clone Wars episode, Obi-Wan discusses his encounter with Jango with Mandalore's Prime Minister Almec. Almec sounds disgusted with Jango's existence, remarking, "Jango Fett was a common bounty hunter. How he acquired that armor is beyond me." Almec went out of his way to distance the Mandalorians from Jango, but "The Tragedy" reveals a clan must have taken Jango in and raised him as one of their own. It clears up a great deal of confusion, as the debate on whether or not Jango and Boba were actually Mandalorians has been a hot talking point within Star Wars fandom for years. The Mandalorian confirms once and for all they are via Jango's status as a foundling. The reasoning for this retcon is unknown, but it's possible Almec was unaware of Jango's history. It's also worth keeping in mind several years separated the productions of the aforementioned Clone Wars episode and The Mandalorian, so it may not have been a plot point Lucasfilm considered back then.
Regardless, shedding light on Jango's past illustrates the benefits of the Star Wars TV shows, as they're able to shed light on fan-favorite characters in ways the films never could. In Attack of the Clones, Jango had only a supporting role as a villain and wasn't given that much depth. On The Mandalorian, viewers gain more appreciation for Jango and Boba, particularly since the episode draws a direct parallel between them and Din. It will be interesting to see if Jango's time with the Mandalorians is explored in canon at some point. The Mandalorian Civil Wars sound like they'd be ripe for a novel, comic series, or even a video game. It would arguably be a wasted opportunity if this thread is never followed up on, but the bits of info in The Mandalorian still go a long way to make the Fetts more well-rounded characters.