The new musical “Summer of ’42” replaces “Reefer Madness” at the Variety Arts, with the latter show closing Oct. 28 after 48 performances.Mitchell Maxwell, lead producer on “Summer,” broke legit etiquette last week when he announced his show’s Jan. 10 opening to group-sales marketers. The producers of “Reefer Madness” did not announce its closing until Wednesday. “Summer of ”42nd” had its debut in fall 2000 at Goodspeed’s Norma Terris Theater in Connecticut. The score is by David Kirshenbaum, with book by Hunter Foster. Gabriel Barre directs.