According to January Jones' statement, it sounds like the character simply won't appear at all. So, fans of the actress don't have to worry about a replacement. Kitty Pryde? There's a character that has been played by three different actresses in three different films, so it's not beyond 20th Century Fox to recast Mutants.
X-Men: First Class stars Jennifer Lawrence and Nicholas Hoult are confirmed to return, along with Hugh Jackman, who had a memorable five minute cameo in the first prequel. Bryan Singer is working from a screenplay by Simon Kinberg and Matthew Vaughn. He directed the first two films in the franchise, X-Men and X2: X-Men United.
X-Men: Days of Future Past was released May 23rd, 2014 and stars Hugh Jackman, James McAvoy, Michael Fassbender, Jennifer Lawrence, Halle Berry, Nicholas Hoult, Anna Paquin, Ellen Page. The film is directed by Bryan Singer.