C525 FLC BECOME DISTR WITH RWY CHANGE AT LAS AND ARE UNABLE TO COMPLETE FMS REPROGRAMMING PRIOR TO THE ACFT EFFECTING HDG CHANGE FROM PREVIOUS PROGRAMMED FLT PLAN RTE.

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DEPARTED KTRM ON A PLANNED FLT TO KLAS WITH 4 PAX. BOTH MYSELF (PIC) IN THE L SEAT AND SIC ARE TYPE RATED (MULTI-CREW) IN THE ACFT. I ELECTED TO FLY THIS LEG. BEING UNABLE TO PICK UP OUR FILED IFR CLRNC USING THE ACFT RADIO; WE DEPARTED VFR. AFTER TKOF; THE SIC WAS SUCCESSFUL IN CONTACTING PALM SPRINGS APCH; AND COPIED OUR CLRNC. WE WERE SURPRISED TO RECEIVE VICTOR AIRWAY ROUTING AFTER HAVING FILED JET AIRWAYS. HAVING PREPROGRAMMING THE GPS WITH OUR FILED ROUTING; WE THEN HAD TO DETERMINE OUR NEW ROUTING AND REPROGRAM THE GPS. ON THIS RELATIVELY SHORT FLT; THIS PROVED TO BE SOMEWHAT OF A TASK. IN MY HASTE; I ENTERED THE CLRED ARR; SKBER RNAV ONE; WITH THE RWY 1; BOTH; NOT REALLY KNOWING WHICH RWYS WOULD BE IN USE. AS WE APCHED LAS VEGAS; THE ATIS INDICATED THAT ILS 25L WAS IN USE. ATC ISSUED AN ALT ASSIGNMENT OF 13000 FT AND I STUDIED THE ARR TO FIND THAT IT CONTAINED A DESCENT PROFILE; IN THAT WE WERE TO CROSS FIXES AT PREDETERMINED ALTS. NOT HAVING BEEN GIVEN THE PROFILE; AND TOLD TO MAINTAIN 13000 FT; I WAS LEFT WONDERING WHY WE HAD BEEN GIVEN AN ASSIGNED ALT. HAVING BECOME FIXATED ON THIS; I NEGLECTED TO CHANGE THE RWY ASSIGNMENT. AS WE NEARED LAS VEGAS; WE TURNED OFF THE STAR FOR THE EXPECTED RWY; AND WERE IMMEDIATELY ISSUED VECTORS. ATC QUESTIONED US AS TO OUR UNUSUAL TRACK AND WE WERE UNSURE AS TO WHY IT HAD OCCURRED. DUE TO MY FIXATION WITH THE DSCNT PROFILE AND INADEQUATE RECHKING OF THE RTING WE DEVIATED FROM OUR CLRED COURSE.