From Deadline:

Matthew Vaughn has decided not to direct the sequel to 20th Century Fox‘s hit spinoff X-Men: First Class. Ready for the next shocker? Atop the studio’s replacement short list is Bryan Singer, who launched the X-Men feature franchise with the first two superb films and who has been a guiding influence in this spinoff by writing the treatment and producing. If this works out, Singer and Vaughn would essentially be switching roles. (Bryan Singer, Cinema, Matthew Vaughn, X-Men: Days of Future Past)