Every so often, DC Comics tends to change the history of the Justice League, usually by adjusting the line-up of the team's founding members. In the Justice League of America's first appearance in The Brave and the Bold #28, Superman and Batman were not involved. The pair was later retroactively added in as founding members, and then taken back out in JLA: Year One as part of Post-Crisis continuity. In 2011, the New 52 changed things again, swapping out Martian Manhunter for Cyborg as a founding member. And that's how the team has stayed -- until now.
Christopher Priest has only been the writer on Justice League for four issues but he's already making a massive change to team history, even if no one has directly acknowledged it yet. J'onn J'onzz has not been seen since the beginning of Rebirth in 2016, but he has finally made an appearance in Justice League #37 (by Christopher Priest and Philippe Briones) in the form of a flashback.
In part four of "The People vs. The Justice League" storyline, the previous issue's Batman imposter is revealed to be a fan of the Justice League who is trying to "help" them by killing their enemies. As he prepares to pose as a Green Lantern, he tells a captured Simon Baz about his interaction with the Justice League as a kid. According to the story, he once witnessed the team battle the Shaggy Man when the lineup consisted of Superman, Batman, Wonder Woman, the Flash, Green Lantern (Hal Jordan)... and the Martian Manhunter.
While he's never actually mentioned in dialogue, it is clear that he is a part of the team and Cyborg is not. This isn't exactly the first hint of Manhunter's involvement with the Justice League, but it is the first time we've seen him in action.
In The Flash #21, part of "The Button" crossover with Batman, Barry Allen goes to the Watchtower to use the Cosmic Treadmill. While he's there he enters a room filled with artifacts from the team's history, which includes the Martian Manhunter's costume propped up in the back. While we never receive any kind of context about his involvement with the League, it is established there that he has at least interacted with them at some point, apparently in a significant enough way to warrant inclusion in the trophy room.
Priest himself hinted at J'onn's newly retconned JL history in Justice League #35, when the bounty hunter Glenn Gammeron made reference to Martian Manhunter, who he once worked with in Justice League Task Force back in the early '90s. Batman tells him that they haven't seen J'onn J'onzz in a long time, so it's possible that there is some untold story waiting to be written by Priest.
What Does This Mean For Cyborg?
The idea of Martian Manhunter being a founding member of the Justice League again flies in the face of the team's previous origin as told in the first six issues of the 2011 title by Geoff Johns. In that story, Superman, Batman, Wonder Woman, the Flash, Green Lantern, Aquaman and Cyborg team up to take on Darkseid during his initial invasion of Earth. It was used to establish the origin of Vic Stone, and served as the basis for the Justice League movie, which has Cyborg among its members.
If the Martian Manhunter is now a founding member of the team again, when exactly did Vic Stone become Cyborg? His origin has been tied distinctly to the forming of the Justice League, and having him join at a later date changes things drastically. It's important to note that the New 52 and Rebirth are still the same continuity, even though some changes have been introduced. If the League's history is different now, there's obviously an explanation that Priest and company plan to share.