There’s a change in the cast of “Black Mass,” the movie about Whitey Bulger. We got word Wednesday that Aussie actor Guy Pearce, who was believed to be playing the mob boss’s law-abiding brother Billy Bulger, has been replaced by Benedict Cumberbatch. And we’re OK with that. The 37-year-old English actor is best known for playing Sherlock Holmes in the TV series “Sherlock” and Julian Assange in “The Fifth Estate.” It’s not clear why the switch was made. The movie, which is being directed by Scott Cooper and stars Johnny Depp as Whitey, was shooting Wednesday in Cambridge. Judging from the graffiti spray-painted on a building (“IRA Brits Out”) and an older model Land Rover with Northern Irish-style plates, the scene was set in Northern Ireland.