A LATE RWY CHANGE; SLOW FMC RESPONSE AND THE NEED FOR MULTIPLE ARRIVAL SEGMENT RE-PROGRAMMING ON THE RIIVR STAR TO LAX RESULTS IN TRACK AND ALT DEVS AND THE NECESSITY FOR RADAR VECTORS IN LIEU OF RNAV FOR A300 FLT CREW.

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APPROX 200 MI FROM LAX WE PLACED THE RIIVR1 ARR FOR RWY 25L IN THE FMS. THERE WAS A LOT OF TFC AND THE FREQS WERE CONGESTED. AS WE BEGAN THE ARR JUST OUTSIDE OF GRAMM WE WERE TOLD TO DSND VIA THE ARR. THEN WE WERE TOLD TO CONTACT SOCAL APCH IN THE VICINITY OF RUSTT INTXN. UPON CONTACT WITH SOCAL APCH WE WERE GIVEN RWY 24R. WHEN GIVEN THE NEW RWY THE CAPT (PM) BEGAN TO PROGRAM IT INTO THE FMS. HAVING TO SCROLL THROUGH THE DIFFERENT ARRS AND TRANSITIONS IN THE DATABASE AND THE SLOW SPD OF THE SCROLL FUNCTION IT TOOK TIME TO SELECT THE PROPER TRANSITION AND RWY. WHILE DOING SO THE FO (PF) BEGAN A SLOW DSCNT TO CONTINUE THE ARR. THE FMS WOULD NOT ACCEPT THE TRANSITION TO THE NEW RWY. APPROX 1 MI FROM RIIVR INTXN A 3RD ATTEMPT WAS TRIED AND REJECTED. ATC ADVISED US THAT WE SHOULD BE ON THE N COMPLEX OF THE ARR. WE ADVISED ATC THAT WE WERE TRYING TO UPDATE THE FMS TO THE PROPER TRANSITION. AT THIS TIME HE INSTRUCTED US TO TURN TO HDG OF 270 DEGS TO INTERSECT THE RWY 24R LOC. AFTER COMPLYING WITH THE INSTRUCTIONS; WE WERE GIVEN A DSCNT TO 10000 FT. WE WERE ALREADY DSNDING BELOW 10000 FT AND BEGAN A CORRECTIVE ACTION TO RETURN TO 10000 FT. DURING THE CORRECTION ATC INSTRUCTED US TO CLB AND MAINTAIN 10000 FT. AFTER LEVELING OFF AT 10000 FT AND ON THE LOC ATC CLRED US FOR THE APCH AFTER THE INTXN PALAC. AFTER PALAC WE WERE GIVEN THE VISUAL TO RWY 24R AND LANDED UNEVENTFUL. WHILE TAXIING TO THE RAMP WE WERE TOLD TO CONTACT ATC VIA A LAND LINE. ATC MADE NO MENTION OF SEPARATION CONFLICT.