Originally Posted by psylockolussus
Are you even sure they are gonna alter the events of Last Stand? The event that the present X-Men wants to alter might have happened post X3, the time where the Sentinels started to pop-up. So if they managed to alter the future, it would be just post X3 and Jean Grey/Cyclops would still be dead and they could continue post X-3 with X4 after DOFP is released.
The event that they're trying to prevent takes place in 1973. By going back and preventing said event the timeline changes, eliminating all the events of the original trilogy. Those events now only exist in an alternate reality.