China has become such an important market for U.S. entertainment companies that one studio has taken the extraordinary step of digitally altering a film to excise bad guys from the Communist nation lest the leadership in Beijing be offended. When MGM decided a few years ago to remake " Red Dawn ," a 1984 Cold War drama about a bunch of American farm kids repelling a Soviet invasion, the studio needed new villains, since the U.S.S.R. had collapsed in 1991. The producers substituted Chinese aggressors for the Soviets and filmed the movie in Michigan in 2009. But potential distributors are nervous about becoming associated with the finished film, concerned that doing so would harm their ability to do business with the rising Asian superpower, one of the fastest-growing and potentially most lucrative markets for American movies, not to mention other U.S. products. As a result, the filmmakers now are digitally erasing Chinese flags and military symbols from " Red Dawn