Total Film (via CBM), screenwriter Simon Kinberg (X-Men: The Last Stand) has revealed the thinking behind the switch, and it's not down to the fact that Wolverine is the crown jewel in Fox's X-Men franchise... well, not entirely, anyway...
"We made the decision for a lot of reasons, some of them obvious and some of them more nuanced, to make it Wolverine who goes back in time. One reason is that he's the protagonist of the franchise, and probably the most beloved character to a mass audience. Probably the bigger reason is that when we started thinking about the logistical realities of Kitty's consciousness being sent back in time, to her younger self, as opposed to her physical body being sent back..it was impossible. Obviously in the book it's Kitty..but you're talking about an actress [Ellen Page] who, in the age of Michael Fassbender [Magneto] and James McAvoy [Professor X], would have been negative 20 years old. So we started thinking again, and the first reflex response to that was a character who doesn't age. Wolverine is the only character who would looks the same in 1973 as he does in the future."
X-Men: Days of Future Past is set for release on May 23rd 2014, with a cast that also includes X-Men veterans Patrick Stewart (Professor X), Ian McKellen (Magneto), Jennifer Lawrence (Mystique), Beast (Nicholas Hoult), Lucas Till (Havok), Halle Berry
(Storm), Anna Paquin (Rogue), Shawn Ashmore
(Iceman) and Daniel Cudmore (Colossus) alongside X-Men
newcomers Evan Peters (American Horror Story) as Quicksilver,
Booboo Stewart (The Twilight Saga: Eclipse) as Warpath, Omar Sy (The Intouchables) as Bishop, Fan Binbing (Iron Man 3) as Blink, Adam Canto (The Following) as Sunspot, and Peter Dinklage (Game of Thrones) as Bolivar Trask.