B757 FLT CREW ON SEAVU STAR TO LAX EXPERIENCES TRACK DEV WHEN LATE RWY ASSIGNMENT FROM ATC RESULTS IN FMS RTE DISCONTINUITY OVER CATAW INTXN.

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ZLA CLRED US FOR THE SEAVU1 ARR TNP TRANSITION. NO RWY WAS GIVEN AT THAT TIME. I LOADED THE RWY 24R EXPECTING THE N COMPLEX BECAUSE THE PRIOR WEEK THAT WAS OUR RWY ASSIGNMENT. LAX THEN CLRED US DIRECT TO KONZL INTXN DSND AND MAINTAIN 17000 FT. STILL NO RWY ASSIGNMENT WAS GIVEN. AFTER PASSING KONZL AT 17000 FT I NOTICED THAT ATC STILL HAD NOT CLRED US ON THE PROFILE DSCNT. CONCERNED WE MIGHT NOT MAKE THE PROFILE ALTS; I REMINDED ATC THAT WE WERE MAINTAINING 17000 FT. AT THIS POINT I BELIEVE WE WERE PAST TRTLE INTXN. AT THAT TIME ATC CLRED US FOR PROFILE DSCNT FOR RWY 25R. I VERIFIED WITH ATC RWY 25R AND IT WAS CONFIRMED. AT THIS POINT WE WERE APPROX 1.5-2 MI FROM CATAW INTXN. AT CATAW THE ARR MAKES A R TURN TO 278 DEGS TO SEAVU INTXN. I IMMEDIATELY RESELECTED THE ARR THEN THE NEW RWY 25R. I NOTICED THERE WAS A DISCONTINUITY ON THE LEGS PAGE. WE FLEW PAST CATAW THE AUTOPLT WENT TO HDG HOLD BECAUSE OF THE DISCONTINUITY ON THE FMC. WE BOTH KNEW WE HAD TO MAKE A TURN AT CATAW. THE FO MENTIONED HE WAS GOING TO START A TURN IN HDG SELECT; I AGREED. I WORKED AS QUICKLY AS I COULD TO CORRECT THE DISCONTINUITY. ATC NOTICED WE MISSED OUR TURN AND GAVE US A VECTOR AND DSCNT FOR RWY 24R ANOTHER RWY CHANGE. ATC CALLED OUT TFC AT 2 O'CLOCK POS; WE SAID 'LOOKING.' ATC SAID 'IF WE DIDN'T HAVE TFC HE WOULD HAVE TO BREAK US OFF THE APCH.' WE SAID WE HAD 'TFC IN SIGHT.' AT NO TIME WAS THERE A TCAS ALERT OF CONFLICT WITH THE TFC. ATC SIMPLY WANTED US TO MAINTAIN VISUAL SEPARATION AND FOLLOW THAT TFC. ATC THEN CLRED US TO INTERCEPT RWY 25R LOC; ANOTHER RWY CHANGE. THE FO PUT THE ILS FREQ IN WHILE I LOADED IT INTO THE FMC AND EXTENDED IT. THE FO FLEW THE ILS RWY 25R AND LANDED WITHOUT FURTHER INCIDENTS. LOOKING BACK ON THIS INCIDENT SEVERAL THINGS COULD HAVE BEEN DONE TO PREVENT THIS. FIRST; I SHOULD HAVE CONFIRMED EARLY WITH ATC WHICH RWY TO EXPECT. SECOND; WHEN THE DISCONTINUITY CAME UP I SHOULD HAVE IMMEDIATELY HAD THE FO SWITCH TO RAW DATA AND FLY THE STAR. PERHAPS; BECAUSE THIS WAS A LONG DUTY DAY 13+ HRS DUTY I MIGHT HAVE BEEN QUICKER IN FIXING THE DISCONTINUITY. PERHAPS; 1.5-2 MI FROM THE CATAW INTXN WAS TOO SHORT OF A TIME FRAME TO FIX THE DISCONTINUITY PROB. THIRD; I PROBABLY SHOULD HAVE NOT CHANGED THE ARR SINCE IT WAS ALREADY IN FMC AND RELOAD ONLY THE NEW RWY ASSIGNMENT. HOWEVER; I WAS UNDER THE IMPRESSION THAT WHEN GIVEN A DIFFERENT RWY THE PNF HAS TO RELOAD THE ARR AND THE NEW RWY. I KNOW ON A SID THAT IS THE PROC. IN SUMMARY THESE EVENTS ARE AS BEST AS I CAN REMEMBER. THE FO AND I TRIED TO PIECE TOGETHER THE CHAIN OF EVENTS AS THEY OCCURRED IN ORDER TO MAKE THE ASAP RPT AS ACCURATE AS POSSIBLE. WE WERE BOTH EXHAUSTED AFTER A LONG DAY AND DID OUR BEST IN TAKING NOTES IN THE TAXI RIDE FROM LAX TO ONT.