AN ACR RECEIVED AN LAX SEAVU RWY CHANGE DURING DSCNT. THE FMS REVERTED TO VNAV SPD AS THE FMS WAS REPROGRAMMED. THE CATAW ALT CONSTRAINT WAS MISSED BECAUSE THE CREW DID NOT SEE THE MODE REVERSION OUT OF VNAV PATH.

Narrative

WE WERE NAVIGATING TO LAX ON THE SEAVU1 ARR. INSIDE OF KONZL; UPON CROSSING KONZL SOCAL APCH CLEARED US TO DESCEND VIA THE SEAVU1 FOR THE ILS TO RWY 24R. I WAS THE NON FLYING PLT AND CHANGED THE APCH IN THE FMC FROM THE ILS RWY 25L TO THE ILS RWY 24R. AT THE TIME OF THE CHANGE THE ACFT WAS ON AUTOPLT DESCENDING VNAV PTH; WHEN THE AIRPLANE REVERTED TO VNAV SPD DURING THE RWY CHANGE. THE CAPT AND I WERE CLARIFYING THE RWY CHANGE WITH EACH OTHER AND I WAS VERIFYING THE LEGS FOR THE TRANSITION AND APCH WHEN I THEN NOTICED THE AIRPLANE REVERTED TO VNAV SPD AND DESCENDED BELOW THE CATAW RESTRICTION OF 14000 FT OR MORE. ACFT DESCENDED AND CROSSED CATAW AT 13500 FT. THERE WAS NO INQUIRY FROM ATC NOR WAS THERE ANY TA ADVISORIES OR RESOLUTION. VNAV PTH WAS REENGAGED AND THE FLT CONTINUED WITHOUT FURTHER INCIDENT. AUTOMATION; WHEN IT WORKS WELL IS A GREAT THING; BUT HAS ITS PITFALLS AS WELL. THOSE OF US WHO FLY IN AND OUT OF LAX FREQUENTLY KNOW THAT ATC WILL CHANGE RWYS ON THIS ARR AT AN INCONVENIENT AND HIGHLY DISTRACTING TIME. BOTH PLTS NEED TO MONITOR ANY CHANGES TO RTE AND VERIFY THAT THE MODIFICATION HAS NOT CHANGED THE PATH AS WELL.