B767 LNDG SFO MISSED INTERCEPT TO RWY 28L AND DSNDED BELOW ASSIGNED ALT DURING ATC RWY CHANGE.

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CAPT FLYING ON AUTOPLT INBOUND ON FMS BRIDGE VISUAL RWY 28R AT 6000 FT INSIDE OF ARCHIE. NORCAL GAVE US A HDG TO NOW INTERCEPT LOC RWY 28L AND DSND TO 5000 FT. THE CAPT CLICKED OFF THE AUTOPLT AS I NOW WORKED THE MCP AND TRIED TO FIND THE NEW LOC FREQ AND LOAD THE BOX; TIME PERMITTING. ALTHOUGH IT WAS VFR; THERE WAS A CLOUD LAYER AT 5000 FT; SO A VISUAL TO THE RWY WAS NOT POSSIBLE. WE WERE KEPT HIGH BECAUSE ATC WANTED TO CROSS AN ACFT BELOW US FROM THE W TO RWY 28R FOR SOME REASON SWITCHING US TO RWY 28L. AFTER FINDING AND DIALING IN THE RWY 28L LOC FREQ; I SAW US DSNDING AT ABOUT 1200 FPM THROUGH 5100 FT AND I CALLED OUT 'ALT.' THE HAND FLYING CAPT LEVELED OFF; BUT NOT BEFORE TOUCHING JUST BELOW 300 FT LOW OF 5000 FT; AT THE SAME TIME OVERSHOOTING SLIGHTLY THE RWY 28L LOC COURSE EVEN THOUGH THE MCP WAS SET UP TO BOTH INTERCEPT THE LOC AND THE ALT. CORRECTIONS WERE MADE QUICKLY AND THE CTLR NEVER MENTIONED IT; BUT ACR Y WAS PASSING IN FRONT OF US AND JUST BELOW US. I BELIEVE THIS PROB WOULDN'T HAVE OCCURRED IF THE AUTOPLT WOULD HAVE BEEN USED THROUGHOUT; AND IT WOULD HAVE BEEN HELPFUL IF THE CTLR WOULDN'T HAVE CROSSED THE AIRPLANES TO OPPOSITE RWYS SO LATE IN THE APCH; INSIDE OF ARCHIE. IT ALSO WOULD HAVE HELPED IF HE HAD QUICKLY GIVEN US THE RWY 28L FREQ TO EXPEDITE LOOKING IT UP.