A B757 WAS GIVEN A LATE RWY CHANGE FROM SLC RWY 34L TO RWY 34R. FOLLOWING FMC REPROGRAMMING AN MCP ALT BELOW THE ALLOWABLE WAS SET CAUSING A LEVEL OFF FOR TERRAIN.

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FLT WAS AT 10000 FT MSL ON VECTORS TO SLC FROM THE S. FO WAS PF. CREW HAD BRIEFED AND SELECTED THE ILS RWY 34L IN THE FMS. APCH CTL ASSIGNED A HEADING TO INTERCEPT THE RWY 34R LOC AND CLRED THE FLT FOR THE ILS RWY 34R. FO REPROGRAMMED THE FMS FOR THE ILS RWY 34R. THE FMS DISPLAY SHOWED INTERSECTIONS FROM SCOER (D15.5 ISLC) INBOUND. ACFT WAS OUTSIDE ATANE (D17.0 ISLC NOT DEPICTED ON MAP). UPON RECEIVING APCH CLRNC; THE FO SELECTED 6100 FT (BESBE ALTITUDE) ON THE AP MODE CTL PANEL; AND FLT LEVEL CHANGE FOR THE DSCNT MODE. FLT LEVEL CHANGE PROVIDED NO PROTECTION FOR SUBSEQUENT ALTITUDE RESTRS ON THE APCH. I WAS VERIFYING THE FMS PROGRAMMING AND ASCERTAINING ACFT POSITION RELATIVE TO ATANE INTERSECTION AS WE BEGAN OUR DSCNT. ACFT WAS AT APPROX 9400 FT SLIGHTLY OUTSIDE ATANE (MINIMUM ALTITUDE 10000 FT MSL) WHEN I DIRECTED THE FO TO PULL UP. ALMOST SIMULTANEOUSLY; APCH CTL CALLED; NOTED OUR ALTITUDE DEVIATION; AND RE-CLEARED US TO 9000 FT AND CLRED US FOR THE APCH. FO HAD LESS THAN TWO MONTHS EXPERIENCE ON THE B757. HIS LACK OF EXPERIENCE AND THE LAST-MINUTE RWY CHANGE CAUSED ME TO BECOME TASK-SATURATED. IN THE FUTURE; I WILL DOUBLY EMPHASIZE CREW COORD AND COM; WHICH COULD HAVE MITIGATED THIS INCIDENT.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT HIS ACFT'S FMC WAS PEGASUS (PIP) UPGRADED WITH A SHORT CUT ALLOWING A DEP/ARR PAGE ARR FIX SELECTED INSIDE OF THE INITIAL APCH FIX; BUT OUTSIDE OF THE FINAL APCH FIX. IN THIS CASE THE FO HAD BEEN ON THE ACFT FOR 1.5 MONTHS FOLLOWING A 4 YR FURLOUGH. AT THE TIME OF THE EVENT; ATC WAS RPTING MODERATE TURB AHEAD AND THE RPTR WAS GETTING THE FLT ATTENDANTS SEATED AND BRIEFING PAX. BECAUSE OF THE OVERLOAD HE DID NOT SEE THE SHORTCUT THAT DROPPED ALT CONSTRAINTS NOR DID HE SEE THAT VNAV WAS NOT SELECTED. IN ANY CASE THE ACFT WOULD HAVE DSNDED BELOW THE CONSTRAINTS BECAUSE THEY WERE REMOVED EVEN IF VNAV WERE ENGAGED. WHEN THE RPTR REALIZED THE ACFT WAS BELOW THE WAYPOINT ALT HE WAS DOING MENTAL MATH ATTEMPTING TO CALCULATE THE ACFT'S REAL POSITION.