Rose McGowan, 27, has been signed to replace Shannen Doherty on the WB show “Charmed,” its producers, Aaron Spelling Productions, have announced. Doherty, who had been fired when she was on Spelling’s “Beverly Hills, 90210,” left “Charmed” after reportedly having clashed with costar Alyssa Milano. (Doherty, 30, was sentenced Tuesday to two days in jail or six days on work furlough and fined $1,500 for her arrest last December for driving drunk near Thousand Oaks, Calif.) McGowan is the ex-girlfriend of shock rocker Marilyn Manson. She was born in Florence, Italy, and was raised in a commune for the Children of God cult. Her first film was 1992’s “Encino Man.” So far there has been no announcement of what McGowan’s character will be called on the show, which is about friendly witches. In all probability, however, it will begin with the letter P, as do all the leading characters’ names. Milano plays Phoebe, Holly Marie Combs plays Piper. Doherty played Prue.