APCH CONTROL DIRECTED RWY CHANGE FOR A320 ON CIVET ARRIVAL TO LAX RESULTS IN LOSS OF MAP DISPLAY AND CONSEQUENT TRACK DEVIATION DURING REPROGRAMMING OF FMS.

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DESCENDING THRU FL180 AND CLEARED TO 'DESCEND VIA' THE CIVET ARRIVAL FOR LANDING ON RWY 25L. AT THIS VERY BUSY TIME WE WERE SWITCHED TO RWY 24R. I QUICKLY CHANGED THE FMGC TO RWY 24R. THIS IS ONE OF THOSE STARS THAT REQUIRE A SPECIFIC RWY 24R CIVET ARRIVAL AND THE FMGC NOT LIKING THE RWY 25L VERSION; REMOVED IT FROM THE FLIGHT PLAN. NOW WITH THE MAP GONE AND STILL TRYING TO MEET ALL CROSSING RESTRICTIONS; THE CAPTAIN USING BACKUP NAVIGATION; MADE A SLIGHT TURN TO THE LEFT. APPROACH SAID THEY WERE SHOWING US SLIGHTLY SOUTH OF COURSE AND WE REPLIED THAT WE WERE CORRECTING; WHILE THE CAPTAIN MADE A TURN TO THE RIGHT. DURING THIS TIME I WAS HEADS DOWN RELOADING THE FMGC. SO NOW WE HAVE IT IN THE BOX CORRECTLY. I PROCEEDED TO FINISH UP THE APCH DESCENT CHECK EXPECTING THE CAPTAIN TO JOIN COURSE -- WHICH HE DID NOT; FLYING NOW TO THE NORTH OF IT. APPROACH THEN ISSUED US A HEADING AND SUBSEQUENTLY DIRECT SKOLL INTERSECTION. THIS WAS A RWY CHANGE ON A VERY BUSY STAR WHILE PASSING FL180 WITH A TWIST. THE TWIST BEING THAT ONE NEEDS TO ENTER THE 'RWY SPECIFIC' STAR AND NOT JUST THE RWY CHANGE OR THE FMGC DROPS IT.