Suspecting that Masterpiece Theatre is showing its age after 36 seasons — an eon in TV years — the program’s producers at Boston’s WGBH will “polish” the brand and expand into new media platforms in order to bring more structure and predictability to the schedule and reach the next generation of Sunday night drama fans. The same courtly theme music by French composer Jean-Joseph Mouret will open the program, but it will lose the little tabletop journey of its video opening and half of the series name. The producers will drop “Theatre” and add headings for three distinct seasonal strands: Masterpiece Contemporary in the fall, Masterpiece Classics in winter/spring and Masterpiece Mystery! (working title) in the summer slot Mystery! now fills.