ALTDEV ALT XING RESTR NOT MET. NON ADHERENCE TO ATC CLRNC IN AN OVERWATER OP.

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DURING 'COAST-OUT' BTWN YQX AND 50N50W I RETURNED TO THE COCKPIT TO RELIEVE THE RELIEF FO. HE HAD RECEIVED THE OCEANIC CLRNC AND SAID IT WAS 'AS FILED.' HE THEN LEFT THE COCKPIT TO RETRIEVE THE FO WHO WAS OBSERVING CREW REST. WHILE ALL OF THIS WAS OCCURRING I WAS GIVEN A FREQ CHANGE FROM GANDER CTR TO GANDER OCEANIC; AT THE SAME TIME COMING UP ON 50N50W I WAS PREPARING THE INITIAL POS RPT FOR GANDER OCEANIC. IMMEDIATELY AFTER PASSING 50W OCEANIC SAID TO SWITCH BACK TO GANDER CTR. WHEN I DID THE CTLR TOLD ME WE HAD BEEN CLRED TO CROSS 50W AT FL290. I CHKED THE FLT CTL PANEL WHICH READ 29000 FT AND ASSUMED I WAS LEVEL AT FL290. WHEN I CHKED MY FMA IT SHOWED OUR ALT AT 33000 FT WITH 29000 FT PRESELECTED. IMMEDIATELY BEGAN DSNDING TO OUR ASSIGNED FL290. FACTORS CONTRIBUTING TO THIS INCIDENT WERE NUMEROUS. THE PRIMARY FACTOR BEING THE CHANGING OF ROLES AND CREW MEMBERS DURING A HIGH WORKLOAD PERIOD. THE SECOND BEING FOR ME TO CORRECTLY READ THE FMA'S AFTER ASSUMING CTL OF THE ACFT. I FAILED TO HEAR THE RELIEF FO TELL ME WE WERE CLRED TO FL290 YET HE SAID HE REMEMBERED TELLING ME. OTHER CONTRIBUTING FACTORS WHICH COULD HAVE PREVENTED THE INCURSION WOULD HAVE BEEN TO EDIT THE XING RESTR IN THE FMS WITH FMS IN PROFILE MODE WHEN GIVEN THE CLRNC BY THE PF. ALSO; I SHOULD HAVE BEEN PRESENT FOR RECEIPT AND ACKNOWLEDGMENT OF THE OCEANIC CLRNC AND ALL SEAT POS SWAPS SHOULD HAVE BEEN ACCOMPLISHED SOONER. SITUATIONAL AWARENESS WAS ALLOWED TO DETERIORATE DUE TO A LACK OF PROPER CRM TECHNIQUES. SUPPLEMENTAL INFO FROM ACN 344002: WHILE FO WAS RESTING OUT OF COCKPIT AND CAPT OUT OF SEAT TO RELIEVE HIMSELF; I COPIED OCEANIC CLRNC FROM GANDER CTR. WE WERE AT FL330. GANDER CLRED US TO CROSS 50N50W AT FL290 WHICH WAS ABOUT 150 MI OUT. I SET FL290 IN ALT WINDOW BUT DID NOT EDIT IT IN THE FMS. WHEN CAPT RETURNED; I GAVE HIM THE CLRNC. AT THAT TIME; I LEFT THE R SEAT TO WAKE UP FO WHO SUBSEQUENTLY RETURNED TO COCKPIT WHILE I WENT INTO CREW REST. CREW FAILED TO DSND TO FL290 BY 50N50W. PROB AROSE WHEN CAPT NOT PRESENT WHEN CLRNC RECEIVED. CONTRIBUTING FACTOR WAS MY NOT EDITING ALT INTO FMS.