AN ACR CREW DESCENDING TO SLC WAS GIVEN A LATE SID CHANGE WITH A LOW ALTIMETER SETTING AND MOMENTARILY WENT BELOW THE ASSIGNED ALT.

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WHILE DSNDING INTO THE SLC AREA WE WERE GIVEN A DIFFERENT SID THAN WE WERE NAVING ON. WE HAD JUST GOTTEN THE NEW SID OUT AND REPROGRAMMED THE FMC WITH ALL THE NEW XING RESTRS WHEN ATC ALSO ASSIGNED A NEW RWY FOR LNDG. AROUND THIS TIME WE WERE GIVEN A DSCNT FROM THE LOW 20'S TO 17000 FT. THE CAPT INITIATED THE DSCNT USING VERT SPD WHILE STILL LNAV'ING IN ON THE SID. AT ABOUT FL190 I BEGAN ACCOMPLISHING MY DSCNT FLOW. AT THAT TIME WE BEGAN EXPERIENCING MODERATE CHOP AND IT LOOKED AS IF WE WOULD SOON BE DSNDING INTO A CLOUD LAYER THAT LOOKED TO HAVE SOME ROUGH AIR IN IT. THE CAPT MADE A CALL TO THE CABIN CREW TO ASK THEM TO TAKE THEIR SEATS AND REMAINED ON LINE WITH THEM FOR A SHORT WHILE TELLING THEM ABOUT THE EXPECTED RIDES AHEAD. I LOOKED AT MY ALTIMETER VERIFYING IT WAS SET AT LCL ALTIMETER OF 29.37. IT WAS AT THIS TIME WE WERE PASSING THROUGH ABOUT 17200 FT. I LOOKED AT HIS ALTIMETER AND IT WAS STILL SET AT STANDARD. I CALLED 29.37 ON THE ALTIMETER AND HE BEGAN DIALING IT DOWN. ONCE HE REALIZED WE WERE GOING TO LEVEL OFF BELOW OUR ASSIGNED ALT; HE DISENGAGED THE AUTOPLT AND BEGAN CORRECTING OUR FLT PATH. WE DSNDED TO AROUND 16500 FT OR SO BEFORE REVERSING OUR TREND. AT THIS TIME ATC ASKED OUR ALT AND I TOLD THEM WE WERE LATE GETTING THE PROPER ALTIMETER SETTING AND WERE CORRECTING TO 17000 FT. WE THEN LEVELED AT 17000 FT AND REENGAGED THE AUTOPLT. LOADED THE NEW RWY IN THE FMC; BRIEFED THE NEW APCH; ACCOMPLISHED THE DSCNT CHKLIST AND LANDED UNEVENTFULLY. WHEN THE CAPT BEGAN TALKING TO THE CABIN CREW; WE SHOULD HAVE ACCOMPLISHED A CHANGE IN CTLS OF THE ACFT. A SIMPLE; 'YOU HAVE THE ACFT/I HAVE THE ACFT;' WOULD PROBABLY HAVE PREVENTED THIS FROM HAPPENING.