After a 40th-anniversary screening of her classic movie musical “Cabaret” on Thursday night, Liza Minnelli and Alan Cumming were already going over their upcoming stage show. “They were in a corner conferring about their show at Town Hall,” said a spy at an Empire Hotel after-party for “Cabaret.” Minnelli, who won a Best Actress Oscar for the movie, and Cumming, who won a Tony Award for Rob Marshall’s Broadway revival, are teaming onstage March 13 for Minnelli’s 67th birthday. Earlier at the Ziegfeld Theatre, Minnelli, Joel Grey, Michael York and Marisa Berenson spoke about the film with Turner Classic Movies’ Robert Osborne, in advance of its Blu-ray release.