A BA32 ACR FLC IS TOLD TO EXPECT TO MAKE AN APCH TO A RWY THAT WAS UNUSUAL FOR THEIR ARR QUADRANT; BUT THEY PREPARED FOR IT. LATER; AFTER SWITCHING FREQS; THEY WERE CLRED FOR THE NORMAL LDA TO RWY 30L. THIS CAUSED MOMENTARY CONFUSION IN THE COCKPIT; BUT THE FLC WAS ABLE TO COMPLETE THE APCH AFTER CONFIRMING WITH THE CTLR.

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WHEN WE CHKED ON WITH STL APCH; HE TOLD US TO EXPECT ILS RWY 30R. WE WERE EXPECTING LDA RWY 30L COMING FROM S; WHICH IS USUAL. SO WE HAD TO SET UP AND BRIEF FOR AN ILS TO RWY 30R. WE WERE THEN LATER HANDED OFF TO THE FINAL APCH CTLR AT STL. THE NEXT CTLR MADE NO MENTION OF PUTTING US BACK ON THE LDA RWY 30L. THE CTLR TURNED US TO INTERCEPT THE LOC AND CLRED US FOR THE LDA RWY 30L WITH NO WARNING EXCEPT FOR THE APCH NAME IN THE CLRNC. I HEARD LDA RWY 30L AND STARTED TO LOOK FOR THAT APCH PLATE; WHICH I HAD TROUBLE LOCATING. AS I WAS DOING THAT; THE LOC (FOR RWY 30R) CAME ALIVE AND THE CAPT (HE MUST HAVE ASSUMED THEY CLRED US FOR ILS RWY 30R AND NEVER CHANGED RADIOS) TOLD ME IT WAS ALIVE. I INTERCEPTED AND WE WERE TOLD CONTACT TWR. THE CAPT SAID TO TWR; 'WITH YOU ILS RWY 30R.' THE CTLR DIDN'T NOTICE WHAT HE SAID; BUT I DID; AND QUESTIONED THAT CAPT WHICH APCH WE SHOULD BE ON. DURING THE DISCUSSION; THE TWR TOLD US TO TURN L AND JOIN LOC. CAPT ASKED FOR WHICH RWY. THEY SAID LDA RWY 30L WHICH WE INTERCEPTED AND CONTINUED TO A LNDG. CTLRS SHOULD HAVE TOLD US EARLIER THAT WE WERE SWITCHING APCHS. I ASSUMED CAPT SET MY RADIO; BUT HE DIDN'T -- I SHOULD HAVE SET MY OWN RADIOS. WHEN I WAS LOOKING FOR MY CHART; I SHOULD HAVE GIVEN ACFT TO CAPT INSTEAD OF FLYING MYSELF. (MAYBE CLUED HIM THAT WE SWITCHED APCHS.)