A lot of stuff from the X-Men/X2/Last Stand was kind of retconned to all hell anyway, timeline-wise and otherwise, and I think that was probably a good thing. It's a prequel/soft-reboot, I think, and by soft-reboot I think they're waiting to see if this does brilliantly well enough to save the X-Men line to see if they want to go on with this timeline rather than the first one. I mean, Hank McCoy isn't even in the first three, nor are Banshee or Havoc or Moira MacTaggert, and they've definitely changed stuff up with Raven.