A B737-300 FLT CREW DEVIATE FROM ARR TRACK AT LAS FOLLOWING A LATE RWY CHANGE.

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WE WERE ON ARR INTO LAS VEGAS AND HAD MADE THE RESTR OF LUXOR AT 12000 FT AND 250 KNOTS. WE WERE TOLD TO DSND VIA THE ARR FOR RWY 19L. THE LNAV WAS SET UP FOR RWY 25L AND SEVERAL POINTS ON THE ARR HAD BEEN DELETED BECAUSE WE DON'T USE THEM FOR RWY 25. I RESELECTED THE ARR AND INADVERTENTLY SELECTED THE WRONG POINT TO GO TO. NEITHER ONE OF US CAUGHT THE ERROR AND WHEN THE ACFT BEGAN A TURN; IT APPEARED TO BE TURNING IN THE RIGHT DIRECTION. I THEN PROCEEDED TO CHANGE THE RWY IN THE LNAV TO RWY 19L AND WHEN I LOOKED UP; WE WERE STILL IN A TURN. CHKING THE LNAV I SAW THE ERROR AND WAS IN THE PROCESS OF TURNING BACK TO COURSE WHEN APCH CTL INTERVENED AND DIRECTED US TO TURN TO HDG 180 IMMEDIATELY. WE WERE ALREADY IN A TURN IN THAT DIRECTION AND COMPLETED THE TURN TO PRESCRIBED HDG (180). SINCE WE WERE OFF THE STAR; SHE THEN GAVE US A DSCNT TO 10000 FT AND HANDED US OFF TO ANOTHER CTLR WHO VECTORED US TO RWY 19L. LNDG WAS UNEVENTFUL. BETTER COORD OF LNAV INPUTS PARTICULARLY SINCE THIS WAS A -300. THE ERROR WOULD HAVE BEEN READILY RECOGNIZED IN A -700 BECAUSE THE MAP PRESENTATION WOULD HAVE LOOKED INCORRECT. COORDINATE THE INPUT/VERIFY/SELECT.