B727 CREW WAS GIVEN A LAST MIN RWY CHANGE AT STL.

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THE WX IN STL WAS 2700 FT SCATTERED TO 4500 FT BROKEN CEILINGS. WE WERE GIVEN THE ILS APCH TO RWY 30R TO BEGIN WITH. WE WERE AT 5000 FT MSL. BEFORE WE INTERCEPTED THE LOC FOR RWY 30R WE WERE ASKED TO TUNE IN THE LDA FOR RWY 30L AND FLY THAT APCH. I DID NOT HAVE TIME FOR A FULL APCH BRIEFING; SO FO DID THE BRIEF. WE DID THIS CHANGE IN RWYS AS APCH CTL PUT THE CESSNA ON RWY 30R. WE ALSO HAD TO CLB BACK UP TO 5500 FT FOR VERT SEPARATION. EVERYTHING WAS OKAY THEN ON THE APCH. WE WERE IMC AT 5500 FT TO ABOUT 2600 FT MSL AND BROKE OUT IN VISUAL CONDITIONS. WE HAD THE ARPT IN SIGHT A GOOD 8 MI OUT. WE HAD BEEN SWITCHED OVER TO THE TWR CTLR AT THIS TIME. HE CLRED US TO LAND ON RWY 30L. WE SAW THE CESSNA ON RWY 30R. SO I MOVED OVER FROM THE LDA LOC COURSE AND LINED UP WITH CTR OF RWY 30L. I THOUGHT THAT I COULD DO SO WITH ACFT IN SIGHT THAT WAS ON THE PARALLEL RWY. THE TWR CTLR THOUGHT WE WERE TOO CLOSE OBVIOUSLY AND ASKED US TO MOVE L AND REJOIN THE LDA LOC FOR OUR RWY. I THINK IF I HAD MORE TIME FOR AN APCH BRIEFING AND CAUGHT THE NOTES ON THE LDA APCH I WOULD HAVE STAYED ON THE LOC TO JOESS FIX AND THEN MOVED OVER FOR LNDG.