We don't even know what the actual details of the movie are, or how much time the old cast will actually get if they do appear.
If the main story is about preventing some catastrophic event, there's no reason why it can't centre around First Class cast. I mean realistically, FC sequel was never going to be some kind of 2-hour catch-up with the characters just to see how everybody is going and how they're dealing with their issues, like some kind of onscreen fanfic. There would always be some kind of central plot, like the Shaw/Cuba plot in First Class, which took up a considerable part of that movie.