AN ACR CREW APCHING DFW SET THE FMS UP FOR A 17C ARRIVAL. ATC ASSIGNED RWY 17L REQUIRING AN FMS PROGRAMMING CHANGE. CAPT DID FMS WORK AS PF AND MISSED A VECTOR HDG.

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SWITCHING TO DFW APCH; WE HAD RWY 17C SET IN THE FMS. THE CTLR TOLD US THAT WE WOULD BE USING RWY 17L. I STARTED TO SET RWY 17L INTO THE FMS WHILE THE FO PUT IN THE FREQS AND SET UP HIS SIDE FOR THE NEW RWY. THE CTLR THEN GAVE US AN INSTRUCTION TO FLY HDG 240 DEGS AT LEMYN. THE FO READ IT BACK CORRECTLY WHILE I WAS BUSY SEQUENCING THE STAR INTO THE FMS. WHEN ONE WANTS TO SIMPLY SWAP A RWY AT DFW; THE FMS PUTS IN EVERYTHING FROM LIT (LITTLE ROCK) ALL THE WAY TO THE RWY. IT TAKES A FEW MOMENTS TO ERASE THE 6 FIXES TO KARLA AND RESYNC IT. WHILE I WAS DOING THAT I MANAGED TO SPIN IN THE 240 DEG HDG IN THE HDG BUG. BY THE TIME I GOT MY SIDE SET UP I DID NOT REMEMBER THE FIX TO TURN TO 240 DEGS; SO I ASKED THE FO IF 'THAT WAS 240 HDG AT KARLA?' HE SAID THAT IT WAS CORRECT; SO AT KARLA I WENT INTO HDG MODE; 240 DEGS. A FEW MOMENTS LATER THE CTLR CAUGHT OUR MISTAKE AND RECLRED US TO LEMYN; AND THEN TO DRAAK ON THE APCH. NO OTHER ACFT WERE IN ANY DANGER DUE TO THIS MISUNDERSTANDING. SUPPLEMENTAL INFO FROM ACN 764518: WHILE OPERATING AS FO; PLT MONITORING; DURING FLT TO DFW WE DEVIATED FROM OUR ASSIGNED COURSE WHILE FLYING THE BONHAM FIVE ARR. EXPECTING AND HOPING FOR RWY 17C ON THIS SLOW MORNING WE LOADED AND BRIEFED THAT APCH WHILE INBOUND TO THE ARPT. APCHING KARLA WE WERE HANDED OFF TO REGIONAL APCH. CHKING IN WITH APCH WE WERE INSTRUCTED TO FLY HDG 240 DEGS OFF LEMYN FOR RWY 17L. AFTER READING BACK THE CLRNC I ASKED IF WE COULD GET RWY 17C INSTEAD AND WAS CURTLY TOLD NO. I THEN WENT 'HEADS DOWN' AND PROCEEDED TO BEGIN RELOADING THE ARR INTO THE FMS. I SAW THE CAPT LOADING THE NEW RWY SO I LOADED THE NEW LOC FREQ AND BEGAN REVIEWING THE NEW APCH AND SETTING UP MY SIDE. DURING THIS TIME THE CAPT ASKED ME TO RECONFIRM WITH HIM THE 240 DEG HDG AT KARLA. I BELIEVED HE WAS SIMPLY ASKING TO RECONFIRM THE HDG ASSIGNMENT AND I SAID; 'YES HDG 240.' HE BELIEVED I WAS CONFIRMING BOTH THE HDG AND KARLA INSTEAD OF LEMYN AS THE FIX TO INITIATE HDG MODE. AS A RESULT OF THIS BREAKDOWN IN COM WE DEVIATED FROM OUR ASSIGNED COURSE TO COVIE THEN LEMYN AFTER KARLA.