FLC OF AN LGT TURNED ONTO THE WRONG AIRWAY DUE TO NOT CATCHING A PDC CHANGE AFTER THE ACFT FMC WAS ALREADY PROGRAMMED.

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WE WERE IN CRUISE AT FL290 PASSING TEXARKANA VOR. ATC NOTIFIED US THAT IT LOOKED LIKE WE WERE FLYING OUTBOUND ON J131 VERSES J42. HE GAVE US A VECTOR TO INTERCEPT J42 WHICH WAS WITHIN A FEW MI BECAUSE OF ITS PROX TO J131- (057 DEGS VERSUS 046 DEGS). WE CONFIRMED OUR FLT PLANNED ROUTING WITH HIM FOR THE REMAINDER OF THE FLT -- WHICH WAS COMPLETED UNEVENTFULLY. (NO CONFLICT) THE CAUSE OF THE ERROR WAS A SUBSEQUENT CHANGE TO OUR FLT PLAN BY OUR DISPATCHER. THE ORIGINAL FLT PLAN WAS MODIFIED AFTER WE RECEIVED OUR PAPER WORK AND HAD PROGRAMMED THE FMS. THE PDC INCLUDED THE MODIFICATION; BUT HAD THE ORIGINAL FLT PLAN ROUTING AT THE BOTTOM OF THE CLRNC BLOCK. IT HAD BEEN CHANGED APPARENTLY TO ALLOW US TO FLY A NATIONAL RTE PROGRAM ROUTING. ALTHOUGH BOTH THE CAPT AND I RECEIVED THE PDC; WE BOTH FAILED TO SEE THE NOTATION TO CHANGE THE RTE OF FLT. POSSIBLE REASONS FOR THE OVERSIGHT INCLUDE A HISTORY OF VERY FEW IF ANY CHANGES TO FLT PLAN RTES ON THE PDC'S BEFORE; ESPECIALLY FROM DFW; THE ORIGINAL FLT PLAN ROUTING PRINTED AT THE BOTTOM OF THE CLRNC BLOCK (LAST READ -- LAST REMEMBERED); AND SIMPLY NOT ENOUGH ATTN TO THE CLRNC DURING A HURRIED PREFLT OF THE ACFT FOR DEP. SEVERAL GOTCHAS CAN OCCUR WITH PDC'S THAT NORMALLY WOULD NOT OCCUR WITH RAPIDLY DELIVERED CLRNCS. WRONG FLT NUMBER BECAUSE OF 2 DEPARTING FROM SAME GATE AREA; RIGHT NUMBER; WRONG DATE; MISREADING OR JUST NOT SEEING A CHANGE IN ROUTING. NORMALLY YOU READ BACK THE CLRNC OR THE CLRNC DELIVERY SAYS 'CLRNC CHANGED AT FOLLOWS;' ETC.