A B737-700 ROLLED AT THE GATE AFTER SHUTDOWN AND 'CHOCKS IN' SIGN WHEN THE ACFT APPARENTLY MOVED AGAINST SLIPPING CHOCKS.

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AT GATE X IN ZZZ; WE WERE MARSHALLED; CHOCKED (COMPLETE WITH CHOCKS-IN SIGNAL); AND GATE SVCS WERE APPLIED. BRAKES WERE RELEASED AS PER FOM. THE TUG WAS NOT YET HOOKED UP. (I SUSPECT THAT NONE WERE AVAILABLE DUE TO THE LARGE PUSH EFFORT UNDERWAY.) AS PAX WERE DEPLANING; AND WHILE FLT CREW WAS PACKING UP FOR AN UNPLANNED ACFT SWAP; THE 'A' FLT ATTENDANT ALERTLY NOTICED WHAT SHE INITIALLY THOUGHT WAS A SHIFT IN THE JETWAY. SHE STOPPED THE DEPLANING WHILE THE JETWAY WAS REPOSITIONED. SHE TOLD ME ABOUT THE SHIFT; AND I APPLIED THE PARKING BRAKE. AFTER MY ARR AT OUR NEXT ACFT; THIS FLT ATTENDANT CAME TO ME AND SAID THAT THE ACFT HAD MOVED; NOT THE JETWAY. A PROVISIONER JUST AVOIDED BEING KNOCKED OFF HIS PROVO TRUCK. THE ACFT HAD MOVED ABOUT 3 FT AFTER SLIPPING CHOCKS. INTERESTINGLY ENOUGH; THAT FLT WAS A LINE CHK FOR ME; AND THE VERY SUBJECT OF RELEASING BRAKES WAS BROACHED. I NORMALLY PUT BRAKES ON; BUT THIS TIME THE ONLY REASON I RELEASED THE BRAKE WAS AT THE BEHEST OF THE CHK AIRMAN WHO CITED THE FOM. IN MY JUDGEMENT; RELEASING BRAKES UNLESS THERE HAS BEEN HVY USE OF BRAKES WITHOUT THE TUG/TOWBAR BEING CONNECTED TO THE NOSE GEAR; IS ASKING FOR AN ACCIDENT (BY FAA STANDARDS) WHEN PAX; PERSONNEL; AND EQUIP ARE DAMAGED. ADDITIONALLY; A BETTER CHOCK WOLD BE ONE WHERE A SINGLE UNIT CHOCK (2 CHOCKS CONNECTED BY A METAL BAR; ALL IN THE SHAPE OF A 'C') WOULD REDUCE THE CHANCES OF CHOCK SLIPPAGE. NEVER RELEASE BRAKES UNLESS THERE IS A HVY STATIONARY OBJECT HOOKED UP TO THE ACFT.