AFTER LISTENING TO ATIS; A CL65 CREW SET UP FOR ILS RWY 31R INTO JFK BUT ARE VECTORED AND TURNED ONTO THE RWY 31L FINAL.

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AS WE APCHED JFK; THE CAPT COPIED THE ATIS. ATIS INDICATED THAT WE SHOULD EXPECT AN ILS TO RWY 31R FOR ARR (DEPS USING RWY 31L). AS THE PF; I BRIEFED THE ILS APCH TO RWY 31R. A FEW MINS LATER; WE WERE HANDED OFF TO NEW YORK APCH AND BEGAN RECEIVING VECTORS FOR WHAT WE BELIEVED TO BE RWY 31R. THERE WERE NUMEROUS BUILDUPS IN THE AREA AND A LARGE TSTM TO THE W AND NW WHICH CONTRIBUTED TO A HIGH WORKLOAD FOR US AND THE CTLR. PART WAY THROUGH OUR VECTORING SEQUENCE; THE PERSON MANNING THE FREQ CHANGED. AT NO TIME DID I EVER HEAR US ASSIGNED TO A RWY DIFFERENT THAN RWY 31R (IE; RWY 31L). UPON RECEIVING OUR FINAL VECTOR AND APCH CLRNC; WE WERE TOLD TO MAINTAIN 170 KTS UNTIL MEALS. AT THIS POINT; WE BECAME CONCERNED THAT THE PARTICULAR FAF (MEALS) WAS NOT ON OUR APCH PLATE AND THERE WAS ALSO TFC DISPLAYED ON OUR TCASII AHEAD AND (ABOUT 1000 FT BELOW US) TO OUR R. AT THIS POINT; WE WERE SWITCHED TO TWR FREQ AND DUE TO THE FACT THAT THE LOC WAS STILL FULL SCALE TO THE R (HAD NOT INTERCEPTED YET) AND THERE WAS TFC 3 MI TO OUR R; AND BELOW US; THE CAPT QUERIED THE TWR AS TO WHICH RWY WE SHOULD BE LINED UP FOR. TWR RESPONDED RWY 31L. AS WE HAD NOT STARTED A DSCNT FROM OUR INITIALLY CLRED ALT AND WERE NOT ESTABLISHED ON THE LOC FOR RWY 31L (WE WERE SET UP FOR RWY 31R AS THAT WAS OUR EXPECTED RWY AND NOT RWY 31L); WE ELECTED TO BE REVECTORED FOR THE APCH. WE WERE REVECTORED FOR THE RWY 31L APCH AND LANDED WITHOUT FURTHER INCIDENT. NO CONFLICT WITH TFC OCCURRED. FACTORS CONTRIBUTING TO THIS SCENARIO INCLUDE: 1) CHANGE OF CTLRS DURING VECTORING LEADING TO LACK OF COM OF CHANGE IN LNDG RWY. 2) UPON LNDG; THE ATIS HAD CHANGED FROM THE TIME WE RECEIVED IT TO INDICATE LNDG ON RWY 31L&R. 3) WE EXPECTED RWY 31R. 4) TO THE BEST OF MY KNOWLEDGE; WE WERE NOT TOLD TO EXPECT RWY 31L PRIOR TO BEING CLRED FOR THE APCH. SUPPLEMENTAL INFO FROM ACN 592374: WE WERE TOLD TO FLY A HDG TO INTERCEPT THE LOC AND MAINTAIN 3000 FT UNTIL ESTABLISHED AND CONTACT THE TWR AT MEALS. I WAS PNF AND READ BACK THE CLRNC; POSSIBLY READING BACK RWY 31L AS THE RWY.