FLC OF A B737 STARTED APCH PRIOR TO RECEIVING APCH CLRNC DURING STAR ARR.

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FLT WAS FROM SLC TO LAX. ORIGINAL CLRNC WAS FOR CIVET ARR INTO LAX. APCHING CIVIT; WE WERE RECLRED THE MITTS ARR TO RWY 24R AND CLRED DIRECT TO A POINT ON THE ARR; POSSIBLY MITTS; AND TOLD TO DSND VIA THE MITTS ARR. THE CAPT WAS FLYING AND I WAS REPROGRAMMING THE FMS FOR THE NEW ARR. WE CROSSED DENAY AT 8000 FT AS PUBLISHED AND BEGAN THE DSCNT TO CROSS MERCE AT 4000 FT WHEN SOCAL SWITCHED US TO ANOTHER FREQ. I CHKED IN AND CALLED 'PASSING (APPROX) 6500 FT ON THE MITTS ARR.' SOCAL ASKED WHAT ALT WE WERE DSNDING TO AND I ANSWERED 4000 FT AT MERCE. SOCAL REPLIED; 'ROGER; CLRED ILS RWY 24R.' THE REST OF THE APCH AND LNDG WAS UNEVENTFUL. LATER; THE CAPT AND I DISCUSSED WHY SOCAL HAD ASKED THE ALT WE WERE DSNDING TO AND REALIZED THAT WITHOUT PRIOR APCH CLRNC; WE SHOULD NOT HAVE DSNDED OUT OF 8000 FT AT DENAY. NEITHER OF US COULD RECALL WHETHER OR NOT WE'D RECEIVED APCH CLRNC PRIOR TO DENAY; BUT CONCLUDED THAT IN LIGHT OF SOCAL'S QUESTIONING OUR DSCNT ALT; AND LATER APCH CLRNC; WE PROBABLY DID NOT. THE ARR PAGE STATES; 'EXPECT APCH CLRNC AT DENAY;' AND WE BOTH KNEW THAT. WE WERE CAUGHT UP IN THE RUSH TO CHANGE TO THE NEW ARR AND THE HABIT OF ALWAYS GETTING APCH CLRNC PRIOR TO DENAY. I HAD NEVER BEEN HELD AT 8000 FT BEFORE AND 'IN THE HEAT OF THE BATTLE' DIDN'T CONNECT APCH CLRNC TO THE DSCNT AT DENAY. NOTHING FURTHER WAS SAID BY SOCAL APCH AND NO TFC CONFLICT RESULTED.