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 would 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 the 19th printing of Munchkin in May 2010. The updated version of Munchkin Cthulhu was released in September 2010.
See The Great 2010 Munchkin Changeover for more information, including a comprehensive list of the cards that changed.