FLT CREW OF FMS EQUIPPED WDB EXPERIENCE TRACK DEV AND CONFUSION WHEN ATL APCH CTL CHANGES RWY WITHOUT SPECIFYING A MEANS BY WHICH TO INTERCEPT THE ASSOCIATED ARRIVAL.

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WE WERE ON THE ARR TOLD TO PLAN RWY 26R (TWICE). THE ATL ARR (RNAV) IS RWY SPECIFIC. WE WERE INSIDE THE 'CHANGEOVER POINT' WHEN THE CTLR TOLD US TO EXPECT RWY 27L. WHEN THE FO LOADED THE FMC; IT KEPT ATL VOR AS THE ACTIVE WAYPOINT SINCE THE RWY 27L RNAV TRANSITION WAS BEHIND US. I WATCHED HIM PUT RWY 27L IN THE FMC AND DIDN'T NOTICE THE INCONGRUITY. THE ACFT WENT STRAIGHT AHEAD TO ATL AND THEN INTO HDG HOLD. BASICALLY; THE CTLR WANTED US TO REJOIN THE ARR; BUT DIDN'T TELL US HOW. THE FO DIDN'T SEE THE RTE DISCONTINUITY IN THE FMS (PROBABLY SHOULD HAVE) AND I WENT 'HEADS DOWN' TO CHANGE CHARTS. I WAS ALSO TRYING TO VERIFY A DSCNT CLRNC AND COULDN'T BREAK IN THE CONVERSATION. APCH FINALLY ASKED US WHERE WE WERE GOING AND I TOLD HIM WE DIDN'T KNOW. SEVERAL PROBS: 1) THE CTLR CHANGED APCHS PAST THE COMMON POINT ON THE ARR. 2) FO SHOULD HAVE CAUGHT IT WHEN HE LOADED IT. 3) I SHOULDN'T HAVE GONE HEADS DOWN TILL I VERIFIED THE 'FINISHED' FMC PRODUCT. (FO PUT IN EVERYTHING EXACTLY RIGHT AND I SAW HIM EXECUTE IT; BUT SHOULD HAVE SEEN TOO MANY LINES ON THE SCREEN.)