A B757-200 CREW RPTS MISSING LAX SEAVU ONE ALT RESTRICTIONS AFTER BEING DISTRACTED WHILE ENTERING LAST MIN FMS ENTRIES FOR A RWY CHANGE.

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ON SEAVU ARRIVAL INTO LAX RWY 25R. ATC CLRNC TO DSND VIA SEAVU ARR. ACFT IN VNAV PATH. ATC THEN CLRS US TO DSND VIA THE SEAVU ARR TO RWY 24R. AS THESE ARRIVALS ARE RWY DEPENDENT; PF (CAPT) ENTERED THE NEW ARR INTO FMS AND EXECUTED IT. THIS CAUSED THE ACFT TO GO INTO V/S HDG MODES WHILE COMPUTING THE NEW PATCH. THE RESTR WAS 16000 FT AT TRTLE AND THE ACFT WAS DSNDING INTO V/S NOT VNAV. WHEN PF SAW THIS HE DISCONNECTED AUTOPLT; AND LEVELED THE ACFT; RETURNED TO 16000 FT AND REJOINED THE VNAV PATH APPROX 1 MILE LATER. THE ACFT DSNDED ABOUT 5000 FT BELOW THE RESTR BEFORE RETURNING TO 16000 FT. REMAINDER OF FLT WAS UNEVENTFUL. NO KNOWN CONFLICTS OR WARNING OCCURRED. PERCEPTIONS: I THINK THAT THE COMBINATION OF RWY SPECIFIC PROGRAMMING OF THE FMS COUPLED WITH A CHANGE IN ARR RWY BETWEEN TWO ALT RESTR FIXES CAUSED THE PROB. LOOKING BACK IT MIGHT HAVE BEEN BETTER TO ALLOW THE ARR CONTINUE TO THE NEXT POINT; TRTLE; BEFORE EXECUTING THE CHANGE TO FMS. ALSO; THESE PARTICULAR ARRIVALS ARE GIVEN AS DSND VIA BUT THEN ATC CONTINUALLY CHANGES THE REQUIRED AIRSPD WHICH CAN CHANGE THE DSCNT PROFILE SUBSTANTIALLY; CAUSING ONE TO BE TEMPORARILY OFF OF THE VNAV PATH.