TWR CLEARED ACFT FOR TKOF ON OTHER THAN THE ASSIGNED RWY.

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ATIS INFO INSTRUCTED US TO CHK OUR DENSITY ALT; WHICH WE DID (IT WAS +/-10000') AND THAT ALL DEPS WERE USING RWY 17 WITH THE WIND BEING FROM 160 DEGS AT 6 KTS. GND CTL CLRED US TO TAXI TO RWY 17. WHILE WE WERE TAXIING OUT GND CTL ASKED IF WE WOULD NEED TO DO A RUN UP AND IF SO; HOW LONG WOULD IT TAKE. WE ANSWERED 'YES;' AND THAT IT SHOULD TAKE ABOUT 60 SECS (EXCEPT FOR THE RUN UP AND RWY ITEMS OUR PRE TKOF CHKLIST HAD BEEN COMPLETED AT THE RAMP). THEY ADVISED US THAT AN ACR FLT WOULD BE TAXIING OUT AFTER US AND INDICATED IT WOULD BE BEST NOT TO DELAY ITS DEP. AT THE HOLD LINE FOR RWY 17 WE COMPLETED OUR RUNUP AND PRETKOF CHKLIST WITHIN THE 60 SEC TIME LIMIT AND ADVISED TWR 'READY IN SEQUENCE.' TWR ADVISED US TO 'HOLD SHORT FOR LNDG TFC.' WE ACKNOWLEDGED THE HOLD AND WAITED WHILE A HVY JET LANDED AND THE ACR JET TAXIED UP BEHIND US. AS THE LNDG JUST PASSED WE ANTICIPATED A CLRNC FOR AN IMMEDIATE TKOF AND BEGAN PREPARING FOR SAME BY REVVING UP THE ENGS; CLOSING THE WINDOWS AND DOORS PROVIDING US VENTILATION; TURNING ON THE BOOST PUMPS AND RADAR AND NOTING THE TIME. AS ANTICIPATED; TWR ADVISED 'CLRED FOR IMMEDIATE TKOF ON RWY 12.' AT THE WORK 'TKOF' WE RELEASED OUR BRAKES AND BEGAN ROLLING FORWARD OVER THE HOLD LINE. ON HEARING THE BALANCE OF THE CLRNC; WHICH ASSIGNED A NEW RWY; WE STOPPED SHORT OF THE LINE MARKING THE SIDE OF RWY 17; ADVISED TWR WE WERE UNABLE TO ACCEPT THAT CLRNC; TURNED AROUND AND TAXIED BACK BEHIND THE ACR FLT TO AWAIT RESEQUENCING. (PLEASE NOTE THAT AT COS RWY 17 IS 11021' AND RWY 12 IS 8511'. AT OUR WT AND DENSITY ALT WE WANTED THE EXTRA RWY.) AT THIS POINT A NEW VOICE CAME ON THE RADIO AND ADVISED US THAT WE NEVER SHOULD HAVE COMMENCED TO TAXI IF WE COULD NOT ACCEPT THE CLRNC. THE PROB AROSE WITH OUR BEING URGED NOT TO DELAY THE ACR FLT WHILE TAXIING OUT AND THE HEARING AS THE FIRST PART OF AN UNEXPECTED CLRNC THE CLRNC WE WERE EXPECTING (DUE TO THE ATIS AND GND CTL). IN THE PAST IT'S BEEN MY EXPERIENCE THAT WHEN TWR WANTED TO CHANGE RWYS; OR CLR US FROM AN INTXN; THEY WOULD ASK; 'CAN YOU ACCEPT...'