ACR X WRONG PDC HDG AFTER DEP.

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PDC STATED KENNEDY 6 DEP; INITIAL HDG 205 DEGS MAINTAIN 5000 FT. PUSHED BACK AND TAXIED TO RWY 31L. WE WERE IN A LONG LINE FOR TKOF. PRIOR TO TKOF; EACH ACFT WAS GIVEN AN INITIAL HDG OR FIX. AFTER WE WERE CLRED INTO POS TO HOLD; I EXPECTED TO BE GIVEN A HDG OR FIX IF ATC WANTED SOMETHING DIFFERENT FROM THE PDC. WE WERE CLRED FOR TKOF WITH NO FURTHER INSTRUCTIONS. AT ABOUT 2500 FT AND HDG 205 DEGS; ATC ASKED WHAT OUR HDG WAS. WE TOLD THEM 205 DEG ASSIGNED. THEY IMMEDIATELY RESTR US TO 3000 FT AND TURNED US TO HDG 050 DEGS. THERE WAS AN ACFT XING IN FRONT OF US AT APPROX 4000 FT. WHEN WE PURSUED THE MATTER OF HDG; WE WERE TOLD THAT THE RWY HAD CHANGED AND THEREFORE THE HDG CHANGED ALSO. THE PDC SAYS NOTHING ABOUT THE INITIAL HDG BEING TIED TO A SPECIFIC RWY AND WE HAD NO WAY TO KNOW WHICH RWY THEY INTENDED TO USE. SINCE THEY SHOULD HAVE KNOWN OF THE RWY CHANGE AND THEREFORE OUR INCORRECT HDG ASSIGNMENT; TWR SHOULD HAVE GIVEN US CORRECT INSTRUCTIONS PRIOR TO TKOF. HAD DEP BEEN BUSY AND MISSED THE TFC CONFLICT A NEAR MISS OR A MIDAIR COLLISION COULD HAVE RESULTED. WE COULD HAVE QUESTIONED TWR PRIOR TO TKOF BUT SINCE THEY HAD INFORMED OTHER FLTS OF DESIRED HDG I PRESUMED THAT WE WERE TO PROCEED AS CLRED. PERHAPS INITIAL HDGS ON THE PDC NEED TO BE PAIRED WITH THE INTENDED TKOF RWY.