B737-700 FLT CREW IS ISSUED A RWY CHANGE DURING CIVET 5 ARR TO LAX AND HAS A TRACK/HEADING DEVIATION.

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ON APCH INTO LAX; WE EXPERIENCED A COURSE DEV ERROR WHILE FLYING THE CIVET ARR. JUST AFTER RUSTT ON THE ARR; WE WERE GIVEN A RWY CHANGE TO RWY 25R. THIS EVENT OCCURRED DURING A HIGH WORKLOAD PHASE OF FLT. AS THE PF I SELECTED THE DEP/ARR PAGE ON THE CDU AND INADVERTENTLY SELECTED THE LAS ARRS INSTEAD OF LAX. AS I WAS ALSO PREOCCUPIED WITH MAKING THE NEXT CROSSING RESTR; I ASKED THE CAPT (PNF) TO TAKE A LOOK AT THE CDU AND CORRECT THE MISTAKE. AT SOME POINT; THE MODIFICATION WAS EXECUTED AND WE NO LONGER HAD THE ARR OR RWY FOR LAX. I WENT TO HEADING SELECT WHILE THE CAPT QUICKLY RELOADED THE LAX ARR AND RWY. WE WERE APPROX .5 MILES N OF EDDSO WHEN THE RTE WAS AVAILABLE. WE CORRECTED THE COURSE ERROR AND COMPLIED WITH ALL THE CROSSING RESTRS. SHORTLY AFTER SHELL ON THE APCH; RWY 24R BECAME AVAILABLE AND WE WERE CLRED FOR THE VISUAL AND LNDG ON RWY 24R. DURING HIGH WORKLOADS IT IS CRITICAL TO HAVE BOTH PLTS INVOLVED WITH PROGRAMMING RTE CHANGES. IT MIGHT BE BEST TO HAVE PNF MAKE THE FMC CHANGES; BUT IN ALL CASES; FMC RTE MODES SHOULD BE VERIFIED BEFORE THE EXECUTE KEY IS PRESSED.