The casting call is looking for extras between "16-75" with a "1973 look" - which would place X-Men: Days of Future Past 10 years after Matthew Vaughn's X-Men: First Class - which could explain why cast members like January Jones and Rose Byrne are unlikely to be returning.
During the press tour of X-Men: First Class, Vaughn said that he would love to set the First Class sequel in the 70s and it seems that even without him at the helm, they are following in his vision. He also said that he would have had the JFK assassination involved in some capacity - but the timings don't work out for that.
The casting call also says that "most of the movie is set in 1973" and that they will be required between "April 2013 and August 2013" in Montreal. This would suggest that the majority of the film would be focused on X-Men: First Class timelines with the original X-Men series in the cameo role.
X-Men: Days of Future Past will feature James McAvoy (Professor X), Michael Fassbender (Magneto), Nicholas Hoult (Beast) and Jennifer Lawrence (Mystique), who return from X-Men: First Class to join X-Men alumni Hugh Jackman (Wolverine), Ian McKellen (Magneto), Patrick Stewart (Professor X), Anna Paquin (Rouge), Ellen Page (Kitty Pryde) and Shawn Ashmore (Iceman).
What do you think about X-Men: Days of Future Past being set in 1973?