B727 CREW INBOUND TO BOS WERE APCHING 6000 FT WHEN APCH CTL ADVISED A CHANGE IN RWYS AND APCH PROC. CREW WAS DISTR SETTING EQUIP UP FOR THE NEW APCH AND OVERSHOT THEIR ALT.

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ON OUR FLT FROM LGA TO BOS JUST AFTER DEP; WE HAD A CHANGE OF RTE FROM THE NORWICH 2 ARR TO THE SUUPP 2 ARR. AT ARMUN INTXN WE RECEIVED VECTORS FOR THE ILS RWY 27 AT BOS. WE HAD VARIOUS ALT AND HDG CHANGES FOR APPROX 20 MINS. WE WERE GIVEN A DSCNT TO 6000 FT AND WHEN WE WERE LEVELING AT 6000 FT APCH ADVISED US OF A CHANGE OF RWY AND TO EXPECT VECTORS TO THE ILS RWY 4R APCH AT BOS. I PULLED OUT THE APCH PLATE FOR ILS RWY 4R AND BRIEFED THE FO ON THE IMPORTANT CHANGES TO THE APCH AND THE FO WAS SETTING THE INSTS AS I BRIEFED. AFTER THE BRIEFING; I NOTICED WE WERE DSNDING TO 5700 FT AND CALLED OUT THE DEV. THE FO THEN ADDED PWR AND CLBED BACK TO THE ASSIGNED ALT OF 6000 FT. THE FO COULDN'T RECALL IF HE ENGAGED THE ALT HOLD AT 6000 FT OR IT FAILED TO ENGAGE. THE SO PRIOR TO 6000 FT WAS TOLD TO BRIEF THE PAX ON THE EXTENSIVE DELAY AND WAS DOING SO AT THE TIME OF THE DEV. (NOTE: THERE WAS NO CONFLICT WITH ANY OTHER ACFT.) THE CREW SUGGESTS THAT THE BLOCK IV AUTOPLT INSTALLED ON THE B727 SHOULD HAVE AN ALT HOLD LIGHT INDICATING WHEN THE ALT HOLD FEATURE IS ENGAGED; SO ALL THE CREW MEMBERS CAN EASILY SEE WHEN IT IS ENGAGED. THE ALT HOLD SWITCH IS ON THE CTR PEDESTAL AND AT NIGHT CAN BE DIFFICULT TO SEE ITS POS.