The unthinkable is about to happen. This St. Patrick’s Day, March 17, will mark the final Broadway performances of Nathan Lane and Matthew Broderick in the smash Mel Brooks musical “The Producers.” But rather than put stars in the show, producers have decided that the Lane role, that of flea-bitten theater impresario Max Bialystock, will go to the little-known British actor Henry Goodman (who nonetheless won an Olivier Award — Britain’s Tony — last year for his Shylock in “The Merchant of Venice”), reports The New York Times, which also says that the Broderick role, of timid accountant Leo Bloom, is still up for grabs. The front-runner for the Bloom part, The Times claims, is TV star Steven Weber, remembered for the NBC sitcom “Wings” and currently appearing on ABC’s “Once and Again.” The choices of replacements seem to signify that the show’s real-life producers are confident that their show is name enough to continue the phenomenal ticket sales that the show has generated since it opened a year ago. (The show concerns a backstage plot by two scheming producers to hatch the worst musical ever made so that they can pocket their investors’ money, but things go hilariously awry.) Lane, 46, who won a Tony last year for his Max, and Broderick, 39, were both offered lucrative salary hikes to remain with the show, said The Times, but they cited prior commitments when they replied “no thanks.” Lane, says the Hollywood Reporter, is trying another sitcom, “Life of the Party,” a CBS show about a one-time TV star who lands on a much bigger stage in Washington when he wins a congressional seat. (Lane’s previous sitcom, “Encore! Encore!,” which premiered on NBC in 1998, went nowhere fast.) Names that had been bandied about on Broadway as possible replacements for Lane in “The Producers” included Jason Alexander and John Goodman, said The Times.