CAPT OF ACR LGT ACFT ALLOWED ACFT TO OVERSHOOT ASSIGNED ALT DURING DSCNT. (ALT EXCURSION).

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ALL 3 OF US IN THE COCKPIT HAD OUR 17L AND 18R APCH CHARTS AND THE ARPT TAXI DIAGRAM AVAILABLE. BECAUSE IT WAS NIGHT TIME AND DFW WAS NOT A RPT WE FREQUENTED; THE ILS 17L AND PLANNED LNDG AND RWY CLRING PROC HAD BEEN BRIEFED IN DETAIL; WITH THE 18R CHART AT THE READY 'JUST IN CASE.' THE RADIOS WERE ALL TUNED FOR THE ILS 17L APCH; AND THE ACFT WAS BEING HAND-FLOWN. WE WERE ON A MODIFIED BASE LEG; DSNDING TO 5000 FT MSL QUITE CLOSE IN; HAVING BEEN LED TO EXPECT A VISUAL 'SLAM DUNK.' THE CALL 'OUT OF 6000 FOR 5000' HAD JUST BEEN MADE BY THE PNF AND ACKNOWLEDGED BY THE PF; AND THE PWR WAS STARTED UP FOR THE LEVEL-OFF WHEN ATC SAID 'CHANGE RWYS TO 18R; ADVISE THE RWY IN SIGHT.' WE WERE LESS THAN 10 MI OUT; ALMOST 5000 FT HIGHER THAN THE RWYS. DESPITE ALL OUR SUPPOSEDLY GOOD SENSE; ALL 3 OF US SIMULTANEOUSLY BEGAN TO SWITCH APCH CHARTS; CHK THE RWY 18R ILS FREQ AND GO 'HEAD DOWN;' ALL WHILE WE WERE SUPPOSEDLY HAND FLYING THE ACFT IN A LEVEL-OFF AT 5000 FT MSL. THE ALT ALERT TONE AND LIGHT SHOCKED ALL 3 OF US BACK TO OUR SENSES AS WE PASSED 250- 300 FT BELOW OUR ASSIGNED ALT. EVEN WITH THE IMMEDIATE THRUST INCREASE AND PITCH-UP; OUR INERTIA CARRIED US TO 400 FT LOW (4600 MSL). WE WERE BACK AT 5000 LITERALLY IN SECONDS; AND FORTUNATELY; NO OTHER ACFT WERE IN THE VICINITY. NO ONE FROM ATC MENTIONED ANYTHING; SO WE DO NOT KNOW WHETHER OUR MOMENTARY EXCURSION EVEN REGISTERED WITH THEM. BUT THE 3 OF US IN THE COCKPIT ALL FELT THAT WE HAD BROKEN TRUST WITH EACH OTHER; OUR PAX; AND THE SYS BY FAILING TO MAINTAIN BASIC DISCIPLINE AND FAILING TO ADHERE TO CORRECT PRIORITIZATION OF TASKS AND DIVISION OF DUTIES; THE MOST SIMPLE CRM. ALWAYS; 1 PLT MUST DO NOTHING ELSE BUT FLY THE ACFT. (SUITABLY HUMBLED; WE SAW RWY 18R AND SUCCESSFULLY UNEVENTFULLY LANDED).