Moviegoers anticipating a jolly time at "Exorcist II: The Heretic" should be warned that they can't expect quite as many boffs as folks who attended promptly. Warner Bros. has already substituted re-edited last reels at all 800 theaters showing the movie, and director John Boorman has flown from Ireland to Hollywood to supervise further cuts and inset a voiceover prologue in which Richard Burton discusses exorcism to the accompaniment of scenes from "The Exorcist."

It's openly acknowledge that one of the goals of this belated repair work is to eliminate the most exploisve outbursts of laughter provoked by the original, unexpurgated fiasco. Ironically, the sagging box-office returns for the film will probably sag even further now that the studio is trying to lock the barn door after allowing the horses to escape in the first place.

In the interest of movie history the least the studio could do is entrust prints of the uncut "Exorcist II" with every major film archive in the country.

George Lucas' "Star Wars" spreads out to six suburban locations today - the Academy, Hampton Mall, Laurel Twins, Marumsco, State and White Flint. Since most of these theaters are located outside the Beltway and none is close to the Uptown, there may be little effect on the theater's business. The grosses are declining, somewhat. The third week was the highest - $101,260. The fourth week skidded to $98,989, while the fifth is plummeting towards a mere $90,000.

Local producer-academician Sheldon Tromberg, whose first production, a bucolic comedy called "Teenage Graffiti," is scheduled to reach the Washington area at long last in a few weeks, has announced a new series of 5-weeks seminar sessions in the theory and practice of screenwriting. Call 966-7799 or 244-3770 for details.

The Anacostia Neighborhood Museum has announced four free movie programs for pre-school and elementary school children at 10 a.m. on July 1, 14, 22 and 29.

The opening program will consists of "NuNu and the Zebra" and "Joshua." Children need not be accompanied by adults. For group reservations, call in advance at 381-6731.

Two foreign films that opened to considerable acclaim in New York, the Italian comedy "We All Loved Each Other So Much" and the Oscar-winning "Black and White in Color," a largely Coast, have been added to this week's opening schedule. The former begins an exclusive engagement at the Outer Circle 1 with "Man on the Roof" moving over to the Outer Circle 2. "Black and White in Color" will play exclusively at the K-B Janus 2. The Dupont Circle and Roth's Tysons Corner 5 share a booking of a diverse program of animated and abstract shorts called "Fantastic Animation Festival."