FLC MADE 'PACKS OFF' TKOF DUE TO HVY ACFT WT. RPTR CAPT; FLYING IN R SEAT AS FO; FORGOT TO TURN PACKS ON AFTER TKOF AND CABIN GRADUALLY CLBED TO ABOVE 10000 FT WHEN THE ALT HORN WARNING ALERTED THE FLC. THE ACR HAS PROC THAT ALLOWS SEAT CHANGES AT CAPT'S DISCRETION; BUT THE CAPT HAD NOT FLOWN AS FO FOR 2 YRS AND WAS NOT FAMILIAR WITH THE POS DUTIES.

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SCENARIO WAS NEWLY UPGRADED CAPT IN L SEAT. I; AS CAPT; WAS FLYING R SEAT PERFORMING FO DUTIES. I HAD INADVERTENTLY MISCONFIGURED PACK SWITCHES FOLLOWING TKOF AT HIGH GROSS WT. END RESULT MEANT THE CABIN ALT CLBED AS WE CLBED ALTHOUGH VERY SLOWLY. BOTH OF US; BUT PRIMARILY MYSELF AS PIC; FAILED TO RECOGNIZE WHAT WAS HAPPENING UNTIL WE REACHED OUR FINAL CRUISE ALT OF 31000 FT. WE WERE AT THE TIME; DISCUSSING OUR HVY TKOF; AND THE PAPERWORK INVOLVED. AS SOON AS I RECOGNIZED THE SIT; WE DSNDED BACK TO 24000 FT AND RESET THE SWITCHES. WE NEVER LOST CABIN PRESSURE NOR DEPLOYED THE MASKS. I HAD NOT FLOWN R SEAT IN ALMOST 2 YRS AND THIS WAS MY 4TH DAY IN THAT POS. I WAS BEGINNING TO FEEL COMFORTABLE. NEEDLESS TO SAY THIS MESSAGE WAS DRIVEN HOME COMPLETELY. COMPLACENCY HAS NO PLACE IN THE COCKPIT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: ACFT IDENTED AS B737-200. THE RPTR CAPT'S ACFT HAS A PROC THAT ALLOWS FO'S; WHO HAVE COMPLETED CAPT UPGRADE BUT NOT A LINE HOLDER; TO CHANGE SEATS AND FLY FROM THE L SEAT. PRIOR TO TKOF MANY DISTRACTIONS OCCURRED; ACFT WAS ORIGINALLY OVERWEIGHT WHICH HAD TO BE RESOLVED; WX WAS A FACTOR AND THE FLT RELEASE HAD TO BE REVISED. THEN; THE HVY TKOF WT REQUIRED AN AIRCONDITIONING PACKS OFF TKOF. THE RPTR CAPT HAD NOT BEEN IN THE FO'S SEAT FOR 2 YRS AND ADMITTED TO NOT BEING TOTALLY COMFORTABLE THERE. PLUS; THE FO WAS IN THE L SEAT FOR HIS OWN TRAINING AND EXPERIENCE AS A FUTURE CAPT AND WASN'T WATCHING THE CAPT'S DUTIES IN THE R SEAT. CONSEQUENTLY; THEY FAILED TO TURN THE PACKS BACK ON AFTER TKOF AND THE CABIN GRADUALLY DEPRESSURIZED UNTIL THEY GOT THE 10000 FT CABIN WARNING HORN. AT THAT TIME THEY TURNED ON THE PACKS AND THE ACFT PRESSURIZED NORMALLY. RPTR TALKED WITH HIS ACR CHIEF PLT AND HIS ASSISTANT AFTER THE FLT TO ALERT THEM TO THE POTENTIAL PROB WITH SEAT CHANGES. THE ACR SENT OUT A BULLETIN TO ITS PLTS REGARDING THE SIT AND OUTLINED FACTORS TO CONSIDER BEFORE CHANGING SEATS. SOME OF THOSE FACTORS WERE: ONLY SEAT CHANGE WHEN THE WX IS GREATER THAN 1000 AND 3 OR WHEN THERE ARE NO DISTRACTING SITS THAT COULD INTERFERE WITH SAFETY OF FLT. RPTR SAYS HE WILL BE VERY RELUCTANT TO SWAP SEATS IN THE FUTURE.