B737 OVERSHOOTS ASSIGNED ALT FOLLOWING LAST MIN RWY CHANGE DEN.

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WE WERE ON DSCNT AND VECTORS FOR VISUAL APCH TO RWY 35R AS PRESCRIBED BY ATC. OUR ASSIGNED ALT WAS 13000 FT. AS WE WERE DSNDING OUT OF APPROX 14500 FT; APCH CTL CHANGED OUR RWY TO RWY 35L VISUAL. UPON ACKNOWLEDGEMENT BY THE FO; I COMMENCED ANOTHER APCH BRIEFING AND ASKED THE SO FOR ANOTHER 'APCH CHK.' THESE COMPANY REQUIRED BRIEFS AND CHKLISTS HAD ALREADY BEEN COMPLETED FOR RWY 35R WHEN WE WERE DSNDING OUT OF FL180. OUR COMPANY PROCS REQUIRE A NEW APCH BRIEFING AND APCH CHK IN THE EVENT OF A RWY CHANGE. I HAD BEGUN HAND FLYING WHEN WE WERE DSNDING OUT OF APPROX 17000 FT MSL AND SOON AFTER; WAS ISSUED A CLRNC OF HDG 330 DEGS AND DSCNT TO 13000 FT MSL. THE CHANGE OF RWY CAME AT APPROX 14500 FT MSL AND SINCE THE WX WAS SO GOOD; AND I DID HAVE THE ARPT IN SIGHT; I GAVE THE QUICK APCH BRIEF (RWY 35L) WHILE STILL HAND FLYING. THE 3 OF US BECAME SOMEWHAT PREOCCUPIED WITH OUR VARIOUS DUTIES AND CHKS; AND I DON'T BELIEVE THE ALT CALLS WERE MADE AS WE APCHED OUR ASSIGNED ALT OF 13000 FT. UPON COMPLETION (SO STATING) OF THE SECOND 'APCH CHK' I BECAME MORE FOCUSED ON THE BUSINESS OF FLYING THE ACFT; AND NOTICED OUR ALT WAS NOW 12400 FT MSL! I IMMEDIATELY ADDED PWR SMOOTHLY AND CORRECTED BACK TO 13000 FT MSL. OUR RATES OF DSCNT AND THEN CLB DURING THIS ALT EXCURSION WERE NEVER GREATER THAN 1000 FPM. IN HINDSIGHT; I SHOULD HAVE RECONNECTED THE AUTOPLT UPON RWY CHANGE; AND USED BETTER OVERALL WORKLOAD MGMNT TO ACCOMPLISH OUR TASKS. THE TIMING OF THE ATC RWY CHANGE COULD NOT COME AT A WORSE TIME.