After the original “X-Men” trilogy and the “Wolverine Origins” spinoff prequel, Fox decided to go back to the beginning with “First Class,” a prequel and reboot of the franchise. Now in the sequel to that prequel, time travel has allowed for Stewart and McKellen to reprise their roles, director Bryan Singer announced in a tweet.
— Bryan Singer (@BryanSinger) November 27, 2012
The elder Prof. X and Magneto will be accompanied by their younger counterparts in the film. Michael Fassbender will once again play Magneto, and James McAvoy will be Prof. X once again in what will inevitably be a conundrum of “Inception” proportions.
“Days of Future Past” is being adapted partly from an “X-Men” comic storyline from the early 1980s that moved between a dystopian future where mutants are living in internment camps and a present where mutants try to change things to come, reports Entertainment Weekly. This means that McKellen and Stewart will be the ones traveling back to the 1950s to help Fassbender and McAvoy change the course of history in reverse “Back to the Future” fashion.
Jennifer Lawrence and Nicholas Hoult have also been confirmed to reprise their roles as Mystique and Beast in the “X-Men” sequel-prequel hybrid. Referring back to Singer’s tweet saying “More to come…” EW suggests that Rebecca Romijn and Kelsey Grammer, who previously portrayed Mystique and Beast, could return to the series and interact with their past selves.
It would also be easy to include Hugh Jackman in the upcoming film, given Wolverine's extended lifespan. He also appeared in a brief cameo in "First Class."
“X-Men: Days of Future Past” is slated for a July 18, 2014, release date. EW notes that his puts Ian McKelllen in a head-to-head race with himself in two prequels that day, with “The Hobbit: There and Back Again” also set to open that day.