What was found
(CAN) FLIGHT DISPATCHED WITH PACK 1 INOPERATIVE AND ENROUTE AT FL310 CABIN ALTITUDE BEGAN TO CLIMB APPROXIMATELY 300 FPM. ALL INDICATIONS ON ECAM PAGE WERE NORMAL. CREW REQUESTED A DESCENT TO 13000 FT WHERE PRESSURIZATION STARTED BEHAVING NORMALLY. FLIGHT CONTINUED AT 13000 FT WITHOUT FURTHER INCIDENT. FOUND ACM NR 1 SEIZE. ACM 1 REPLACED IAW AMM 21-52-21, LEAK CHECK & OPS CHK C/OUT IAW AMM 21-63-00. PACK NR 1 SERVICEABLE.