Yep, Patrick Stewart and Ian McKellen will be in the next X-Men movie
As many suspected when it was first announced that X-Men: Days Of Future Past would draw from that story arc's time-travel plot—and when then-producer, now director Bryan Singer all but said as much—the X-Men: First Class sequel will officially square that film's continuity with Singer's original trilogy by having Patrick Stewart and Ian McKellen reprise their Professor X and Magneto roles, respectively. Singer made the announcement via his Twitter account, first welcoming the return of First Class stars James McAvoy, Jennifer Lawrence, Michael Fassbender, and Nicholas Hoult, then confirming that McKellen and Stewart would also appear—a confluence of X-Men young and old about which Singer urges the easily confused to "use your imagination." Or, you know, read the comic, or even just this sentence describing how the comic has a time-travel plot, which we already told you up front. And then, yes, use your imagination, because that's important. Anyway, Singer also added that there is "more to come…," because just writing "also, Hugh Jackman will be in it, because he always is" would be anticlimactic.