"Buffy" did it. "Fringe" did it. Hell, even "Grey's Anatomy" had a musical episode -- so why not "Downton Abbey"?
Granted, the video below is a spoof, but it's a very good one directed by John Walton West ("Newsical the Musical") with music and lyrics by Jason Michael Snow (currently an actor in "The Book of Mormon") and featuring a host of other Broadway stars, including Jeremy Jordan ("Newsies" and, of late, NBC's "Smash"), Jane Blass ("Annie"), Tony Sheldon ("Priscilla Queen of the Desert"), Catherine Charlebois ("Wicked"), Matt Doyle ("War Horse") and others, with comedian Colin Mochrie playing "Downton" creator Julian Fellowes. Alongside finely crafted lyrics like "Edith, let me cleanse this palate / For you, I'll be more than just a valet!" the video pokes fun of the series' tics and tropes as well as former cast member Dan Stevens' departure (of his not participating in the project, "Fellowes" sniffs "I suppose he broke his feet jumping off the bandwagon!").
The actual (and probably non-musical) fourth season of "Downton Abbey" is slated to air this fall in the U.K.