LATE RWY CHANGE RESULTS IN DESCENT BELOW CLRED ALT AT ORD FOR FA50 FLT CREW.

Narrative

ON DSCNT INTO ORD THE FO PICKED UP THE ATIS WHICH ADVISED OF APCHS BEING CONDUCTED TO THE WEST RWYS. THE FMS WAS PROGRAMMED AND I BRIEFED THE RWY 28 ILS. APPROX 25 NM FROM THE ARPT WE WERE TOLD TO TURN R TO INTERCEPT THE RWY 4R LOC; MAINTAIN 8000 FT UNTIL DEGAN; CLRED ILS RWY 4R. WE WERE UNPREPARED FOR AN IMMEDIATE APCH; BUT DID OUR BEST GETTING THE APCH SET UP. WHAT WE DIDN'T KNOW WAS THE GS IS BELOW 8000 FT MSL OVER DEGAN. SO; AS WE ARE TRAINED; I ARMED THE APCH MODE ON THE FGP AS SOON AS THE APCH CLRNC WAS GIVEN AND THE FREQUENCY TUNED. SINCE WE WERE GIVEN THIS RWY CHANGE ON SUCH SHORT NOTICE I LET THE AUTOPILOT FLY WHILE THE FO AND I REVIEWED THE APCH AND RAN THE CHKLISTS. I LOOKED UP JUST AS THE AUTOPLT CAPTURED THE GS AND STARTED A DSCNT FROM 8000 FT. I NOTICED THAT WE WERE ABOUT 2 MILES SHORT OF DEGAN AND TURNED THE AUTOPLT OFF AND RETURNED TO 8000 FT. THANKFULLY; WE ONLY DESCENDED ABOUT 150 FT BELOW THE ASSIGNED ALTITUDE BEFORE RETURNING. I BELIEVE THAT APCH CTLRS SHOULD EITHER WARN CREWS ABOUT INTERCEPT ALTITUDES VERBALLY OR ON THE APCH PLATE IN THIS TYPE OF SCENARIO. I DON'T BELIEVE THAT WE CAUSED ANY CONFLICT; BUT WE MIGHT NOT BE SO LUCKY NEXT TIME.