Let's assume for the moment that Bryan Singer is still coming back to direct X-Men: Apocalypse. If he does, he has an idea of what he wants to do with the cast - and it could be replacing them.
Speaking to SciFiNow (in an interview given before he pulled out from all press after being accused of raping a teen boy) Singer says he has more orgin stories to tell:
“My next goal is how did the X-Men come together? That story hasn’t been told yet, so [Days Of Future Past] is the inbetweequel. It’s in between the birth of that relationship with Charles and Erik and the formation of the X-Men.”
Actually it was told in X-Men: First Class, but I guess he means that he wants to tell the story of how that particular group of X-Men came together in the first film. But to do that, he'd need younger versions of the characters, wouldn't he?
I don't know why everybody is tap-dancing around the fact that Days of Future Past must be a reboot if it's to be of any actual value to the series at all; I haven't seen the film, but the junket press was pretty positive about it (click here for a list of embargo-breaking tweets. First one is from a guy who loved Amazing Spider-Man 2, just so you know), and I'm sure one of them will drop a hint about whether or not the film reboots the series in their pieces.
If they are rebooting (and again, I don't know, I haven't seen it or talked to anyone who has seen Days of Future Past), I would prefer they change the line-up of the team. Singer's X-Men team is so boring and full of lame-ass 90s era mainstays that it bores me to tears. Bring on Forge already.