Anyone who becomes a Yankee is instantly transformed into a s***-eater and douche, if they were so before joing the Yankees, the effect is doubled (see Roger Clemens)
Also, Damon is a rare case as he is the biggest shitbag of all but wasn't before joining the Skankees. His traitorous actions caused him to digest such a large amount of s*** that he was able to have the Clemens effect without even being a douche beforehand.