In May 2010, Steve Jackson Games (the publisher) announced a re-vamp of the Munchkin series. The rules and some of the cards were modified and clarified, and the fantasy sets will be re-done in color. Sometimes referred to as "version 1.5", the publisher was clear that, while the change is significant, it is not a new edition of the game. The re-releases were staggered, starting with Munchkin in May 2010.
See The Great 2010 Munchkin Changeover for more information, including a comprehensive list of the cards that changed.