Date-shifting paid off at the box-office last week; shape-shifting did not. To the surprise of many local exhibitors, Ridley Scott's science-fiction detective thriller BLADE RUNNER did impressive business, after postponing its original opening date a week. At the same time, the science-fiction competition presumed to be commercially safe, John Carpenter's remake of THE THING proved a major disappointment. Since Carpenter seemed to be running an inexplicable winning streak from "Halloween" through "The Fog" through "Escape to New York," it may be a case of the law of averages finally catching up with a slippery target. The irony of this head-to-head encounter, of course, is that many of the horror sensations in Carpenter's film are obviously inspired by a desire to top the grisliest highlights from Scott's "Alien."

To the surprise of no local exhibitors, the ridiculous MEGAFORCE also struck out in its opening weekend and may indeed be headed for television by the time you read these words. The concert film MONTY PYTHON LIVE AT THE HOLLYWOOD BOWL (they appeared there in September 1980) arrives today at area theaters on short notice, inserted by Columbia to plug gaps left by the failure of HANKY PANKY -- a failure that should also discourage development of three more co- starring vehicles for Gene Wilder and Gilda Radner that were supposedly in the works.

DINER adds five more theaters today -- AMC Academy, Roth's Randolph, Roth's Tysons Corner, Showcase Beacon Mall and Showcase Fair City Mall -- while cruising along at the Jenifer. E.T., the out-of-sight commercial pace-setter everywhere in the country, is scheduled to add a few more local auditoriums on July 23. Instead of avoiding competition, the new animated feature THE SECRET OF NIMH, the work of several ex-Disney artists, will confront the great new commercial hope of the Disney studio, the science-fiction, computer-graphics fantasy TRON, next weekend. In fact, the frequency of postponements and altered opening dates may make it useful to update the remainder of the summer release schedule as it currently appears.

Next Thursday brings the Scottish import GREGORY'S GIRL to the Outer Circle. Next Friday TRON and THE SECRET OF NIMH share the spotlight. Woody Allen's A MIDSUMMER NIGHT'S SEX COMEDY, the Kenny Rogers comedy SIX PACK and the soap-opera spoof YOUNG DOCTORS IN LOVE are scheduled for July 16, supplemented by the reissue of RAIDERS OF THE LOST ARK. The 23rd brings THE WORLD ACCORDING TO GARP and THE BEST LITTLE WHOREHOUSE IN TEXAS and offers a faint possibility of THE ROAD WARRIOR, which may now slip back to the fall. AN OFFICER AND A GENTLEMAN has Wednesday, July 28, to itself. On July 30 the comedy NIGHT SHIFT would appear to have the competitive edge over Chuck Norris' FORCED VENGEANCE and the erotic import FRUITS OF PASSION, an exclusive at the K-B Janus. The Jerry Belson satire JEKYLL AND HYDE: TOGETHER AGAIN, originally scheduled for the 30th, has now been postponed to the fall and retitled BABY, IT'S YOURS.

At the moment, it appears that two modest but classy attractions originally intended for the summer, Tim Hunter's TEX and Caleb Deschanel's THE ESCAPE ARTIST, will be reserved for the fall, when they probably stand a better chance of attracting attention anyway.