AFTER ATC SAID TO EXPECT RUNWAY 24R ON THE LAX SEAVU ARRIVAL; 25L WAS ASSIGNED LATER. A TRACK DEVIATION OCCURRED WHEN THE FMC DID NOT ACCEPT THE ARRIVAL AND RUNWAY CHANGE.

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ON APCH TO LAX; WE WERE ASSIGNED RWY 24R. JUST BEFORE SEAVU INTXN WE WERE ASSIGNED RWY 25L. I WAS THE PM SO I SELECTED RWY 25L IN THE FMC. WHEN I LOOKED AT THE NEW INPUT THERE WERE ONLY 3 WAYPOINTS ON THE NEW COURSE. I INFORMED THE PF THAT THIS DID NOT LOOK RIGHT. I RESELECTED THE RWY AGAIN; BUT THE SAME THING CAME UP. AT THIS TIME WE WERE CLOSE TO SEAVU INTXN; SO I HAD THE PM SPELL THE LUVYN INTXN TO ME AND I INPUT IT IN THE FMC. THE AIRPLANE NOW HAD TO FIX TO HEAD TO. I BEGAN TO INPUT ALL THE WAYPOINTS ONE AT A TIME. THE PM WAS FLYING AND DOING ALL THE STEPDOWN FIXES. AT THIS POINT AS I WAS INPUTTING THE FIXES; I SAW THAT THE AIRPLANE DID NOT TURN ON THE COURSE. I INFORMED THE PM THE LNAV HAD KICKED OFF AND TO TURN L NOW. WE MADE THE TURN; GOT BACK ON COURSE; AND SELECTED LNAV AGAIN. AFTER THAT WE DID ALL THE STEPDOWN FIXES WITH DME ON THE LOC. SUPPLEMENTAL INFO FROM ACN 811652: JUST PAST PECOS WE WERE GIVEN RWY 25L. PM BEGAN PROGRAMMING RWY 25L IN THE COMPUTER WHILE I FLEW THE ARR. PM TRIED 2 TIMES TO ENTER RWY 25L BUT WAS MISSING 5 INTXNS AND THE RWY. PM STARTED ENTERING ALL BY HAND WHILE I CONTINUED TO FLY. ON ARR AT SEAVU PM HAD ALL THE POINTS IN AND WE VERIFIED. I THEN VERIFIED WE WERE PROCEEDING TO LUVYN AND CONTINUED THE DSCNT AS PUBLISHED. SOMEWHERE AROUND LUVYN; THE LNAV 'KICKED OFF.' WE IMMEDIATELY TURNED TOWARD KRAIN TO GET BACK ON THE ARR. WE ARE NOT SURE EXACTLY WHAT HAPPENED. ALL WAS WELL BRIEFED AND VERIFIED. THE LNAV WAS RE-ENGAGED ONCE ON COURSE WITH NO FURTHER PROBS. ATC WAS VERY BUSY. WE WERE GIVEN RWY 25L AT THE VERY LAST MIN. WE DID OUR BEST TO GET IT IN QUICKLY. MORE HEADS UP FROM ATC WOULD HAVE HELPED.