After two days on the set of Warner Bros.’ “Demolition Man,” thesp Lori Petty was fired because of “creative differences,” according to producer Joel Silver, who assures the shoot is going well otherwise.“Love Potion 9″ star Sandra Bullock has already been hired to replace Petty (“A League of Their Own”) in what Silver calls his”anti-buddy movie” starring Wesley Snipes and Sylvester Stallone. Bullock’s been spending a fair bit of time on the Warners lot and recently worked on “Wrestling Ernest Hemingway” for producers Todd Black and Joe Wizan. “Demolition Man” is a futuristic action thriller about an unconventional supercop (Stallone) and his sociopathic adversary (Snipes) who are frozen in time for alleged crimes until they are needed in the future. Bullock’s role “is not a gigantic role, but it’s a good part,” Silver explains. “She plays a cop in the future who meets Sly’s character.” Pic is skedded for a fall release.