Your making a bad assumation.First Class,Original films,and Wolverine solo films aren't different series they are all part of same series.
All the first Class actors had multifilms In their contracts.It Is only a question of who they want to come.
Bryan SInger's tweets may have spilled that main focus of DOFP will be on Xavier,Magneto,Mystique,and Beast of First Class past and Xavier,Magneto,and Wolverine In future.
This probally means time traveler will eather be
1:Xavier switching mind with younger self
2:WOlverine switching mind with younger boned clawed self
3:Wolverine physically traveling back In time
Of course If Anna Paquin or less Likely ELlen Page Is next announced as being In film I could be wrong.
It's unlikely that Bryan would tweet every single actor In film.Most likely the ones to watch for bryan to possable tweet are Rose Byrne,January Jones,and Jason Flemying
from First Class,Halle Berry,Anna Paquin,Rebecca Romijn,and possibly James Marsden and Famke Janssen from X-Men and X2 and less likely but still a possibilty Ellen Page
from Last Stand(If any actor cast In a nonsinger X-Men film Is In film It would be Ellen)
If James or Famke or both are included In Bryan Tweet since It's very unlikely Bryan would announce a cameo at end of film we are looking at 2 possibiltys
utright Ignoring the Last Stand
2;Days of future Past Is straight up prequel and time travel preventing the event which leads to sentinles In Dark future leads to original trilogy
2 would allow for bryan's connecting dots between films comments and also would allow him to use James and Famke In film.
It will be Intresting to see If Fox bring DOFP to COmic Con this summer or will they let first look at DOFP be In post credit scene of The Wolverine.