The earlier films still happened(at least the ones they want to Include) something happens between end of first class and whenever first X-men took place that leads
to unleashing the sentinles.I also don't expect Angel or Beast(In part to avoid the why beast looks different Issue) In future sequenze.Part of reason I don't think they will have telephatic contact between both Xavier's Is I think the future will be confined to a 15 to 20 Minute prologue.It's fairly obvious this Is combination of Bryan Singer's X4 Ideas
with first Class sequel.Xavier,Magneto,wolverine,Storm and some combination of Rogue or Kitty and Iceman or Colossus are likely to the returning characters In future prologue.
Preventing what originally caused the sentinles to be unleashed being prevented has
2 added benefets.
1:In third film they can Introduce teenage cyclops and Jean without having to even address teenage cyclops as seen In Wolverine and young jean at begining of Last Stand.
2:It opens the door for Singer to direct a film based on his x5 Ideas after this trilogy without having his hands tied by what was done In The Last Stand.
I expect just like end of Days of future past original storyline In Comics at end of film you won't know If the future was prevented.