CITATION CE-560XL FLT CREW HAS A TRACK/HDG DEV DURING ARR TO LAX FOLLOWING A CLOSE-IN RWY CHANGE.

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WHILE ENRTE FROM ELP TO LAX; WE WERE FILED FOR THE PDZ 4 ARR AND BRIEFED THE ARR. THE PF WOULD STAY ON L NAV AND THE PNF WOULD BRING UP THE LOC ON GREEN NEEDLES; EXPECTING RWY 25R. FROM DIRECTION OF FLT; I ALSO PULLED THE CHART FOR RWY 24R. APCHING PDZ WE WERE TOLD TO FLY THE SEAVU 1; EXPECT RWY 24R. I STARTED TO SET UP RWY 24R BUT NEVER BROUGHT UP THE RWY 24R LOC FREQ. I COULD NOT GET THE SEAVU 1 ARR TO LOAD AND HAD TO BUILD IT BY HAND ONE WAYPOINT AT A TIME. THIS KEPT MY HEAD DOWN IN THE BOX FOR A LONG TIME; JUST KEEPING UP WITH CROSSING RESTRICTIONS. ATC ASKED IF WE HAD INTERCEPTED THE RWY 24R LOC. I SAID YES. ATC TOLD US TO FLY HDG 270 AND INTERCEPT THE RWY 24R LOC. AT THAT TIME I KNEW WHAT WAS WRONG AND FLIPPED THE RWY 24R LOC FREQ UP AND WE INTERCEPTED AND FLEW THE ILS AND LANDED. ATC NEVER ASKED US TO CALL THEM AFTER LNDG. TO PREVENT A REOCCURRENCE; I WOULD SUGGEST THE FOLLOWING: 1) NEVER HAVE TWO APCH PLATES OUT AT THE SAME TIME TO PARALLEL RWYS. 2) ADVISE ATC IF YOU NEED MORE TIME TO LOAD A NEW ARR. 3) VERIFY AND IDENTIFY YOUR LOC.