ACR LGT ALT DEV EXCURSION FROM CLRNC ALT. STARTED APCH BEFORE BEING CLRED.

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APCH INTO DALLAS IN LGT; LATE DSCNT ON DOWNWIND WITH 'EARLY' TURN TO BASE MADE US A 'LITTLE' HIGH. WE WERE TOLD TO EXPECT 17L. SEEING WE WERE GOING TO BE HIGH; WE HAD MAX DRAG OUT. WE NOTICED THAT APCH WAS GOING TO GIVE US A LATE TURN TO THE SW TO INTERCEPT 17L LOC. WE WENT THROUGH LOC WHEN APCH CALLED (HE SEEMED TO BE 'OFF' ON ALL HIS CALLS; SAID CLRED TO 4000 FT (AT 6000 FT AND HIGH!) AND CHANGED RWY TO 18R. THEN GAVE US TURN TO SW TO INTERCEPT LOC. I THOUGHT WE WERE CLRED FOR APCH AND SO FLEW IT. AT 3600 FT AND STILL HIGH HE CALLED US CLRED FOR APCH. THERE WAS NO TFC ANYWHERE NEAR US AND WE WERE COMING DOWN WITH FULL BOARDS AND GEAR; NEVER GOT ON GS UNTIL APPROX 800 FT. WELL; I LOOKED AT OUR ALT ALERT (4000 FT). WERE NOW AT 3400 FT AND I'M THINKING (APCH SELECTED ON MCP). WELL; IF HE HAD NOT CLRED US FOR APCH EARLIER THEN I BUSTED ALT BY 400 FT. TO BE HONEST; WE WERE SO HIGH AND SO BUSY WITH LAST MIN CHANGES THAT I CAN'T RECALL THE CTLR'S EXACT WORDS WHEN GIVEN THE SW HDG FOR 18R (____ 17L). I HAVE BEEN FLYING INTO DALLAS FOR 7 YRS AND THIS LAST MIN HURRIED CHANGE OCCURS WAY TOO OFTEN AND CONSIDERING TFC ON BOTH SIDES OF THE ARPT AT THIS TIME; NO NEED FOR THIS CHANGE. WHAT BOTHERS ME IS HOW BOTH CTLR AND PLT ARE BEING SET UP FOR MISCOM IN THE MOST CRUCIAL/BUSY TIME OF A FLT AND IN THIS ONE INSTANCE; I JUST CAN'T REMEMBER APCH'S PRECISE WORDING. I HOPE THIS LETTER ADDS TO THE PILE YOU GET ABOUT DFW AND MORE IMPROVEMENTS ARE MADE.