ENRTE FLC CREW ISSUED CLRNC TO CLB FROM FL240 TO FL270. DURING CLB; ATC ISSUES CLRNC TO CROSS 50 NM FROM SWEED AT FL240. FO RESET ALT PRESELECTOR BACK TO FL240 WITHOUT NOTIFYING CAPT.

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OPERATED FLT TO CVG. THERE WAS A LINE OF FAST MOVING SEVERE TSTMS MOVING FROM W TO E; STARTING APPROX 100 NM S OF SWEED INTXN AND ENDING APPROX 20 NM N OF FLM; WITH TOPS AT APPROX FL270. WE WERE FILED TO FL240. AFTER LEVELING AT FL240; WE REQUESTED FL270 AND THE REQUEST WAS APPROVED. WE WERE ENCOUNTERING LIGHT TO MODERATE TURB AND PAINTING LEVEL 3-5 RETURNS. WE WERE MAKING SLIGHT DEVS E AND W OF COURSE TO AVOID WX. DURING THE CLB TO FL270; AT APPROX FL252; ZID ISSUED A CLRNC TO CROSS 50 NM S OF SWEED AT FL240. I PLOTTED THE 50 NM FIX S OF SWEED INTO THE FMS AND THIS FIX WAS IN THE MIDDLE OF THE MOST SEVERE RETURNS ON THE RADAR. AT THIS TIME WE STARTED ENCOUNTERING CONTINUOUS MODERATE TURB WITH FREQUENT LIGHTNING FLASHES. I TOLD THE FO TO REQUEST WE NOT START OUR DSCNT UNTIL PASSING THE 50 NM S OF SWEED FIX AND ATC REPLIED; 'UNABLE DUE TO TFC INBOUND TO CVG.' AT SOME POINT FROM THE INITIAL ISSUANCE ON THE ALT XING OF FL240 AND ABOVE FL250 AND BELOW FL260; THE FO RESET THE ALT PRESELECTOR TO FL240 WITHOUT NOTIFYING ME. I WAS UNAWARE THAT HE HAD CHANGED IT FROM FL270. SINCE WE WERE MORE THAN 1000 FT FROM BOTH FL240 AND FL270; A 1000 FT WARNING WAS NOT ISSUED BY THE ALERTER. AT FL266; EVERYTHING WAS STABLE AND I WAS ANTICIPATING THE AUTOPLT TO LEVEL IN ABOUT 20 SECONDS. I AGAIN WAS LOOKING AT THE RADAR AND TRYING TO DETERMINE WHICH WAY WAS THE SAFEST WAY TO DEVIATE FROM COURSE TO COMPLY WITH ATC'S XING. AFTER ABOUT 20 SECONDS; I XCHKED INSTS AGAIN AND NOTICED THE ACFT CLBING THROUGH FL272 AT AN ACCELERATED RATE OF CLB DUE TO A SUDDEN INCREASE IN TURB. I IMMEDIATELY DISCONNECTED THE AUTOPLT AND TRIED TO ARREST THE RATE OF CLB. THE TURB WAS SEVERE AT THIS POINT FOR ABOUT 5-10 SECONDS AND THEN SUBSIDED. THE ACFT WENT BTWN 600-700 FT ABOVE FL270 AND WE WERE ABOVE FL270 FOR APPROX 10-15 SECONDS TOTAL TIME. I MANUALLY ESTABLISHED THE ACFT AT FL270 AND RE-ENGAGED THE AUTOPLT; WHEN I NOTICED THE ALT PRESELECTOR HAD BEEN CHANGED TO FL240. SUPPLEMENTAL INFO FROM ACN 542891: BOTH OF US WONDERED WHY; AND IT BECAME OBVIOUS THAT I HAD SET THE PRESELECTOR WITHOUT CONFIRMATION. WE WERE TAUGHT IN CL65 GND SCHOOL ABOUT THE DANGER OF CHANGING COMMANDED ALTS AND DIRECTIONS BEFORE REACHING THE FIRST ASSIGNMENT. DUE TO THE LAW OF INTENSITY I WILL NOW REMEMBER THIS LIMITATION -- PERIOD. IT EASILY WOULD HAVE BEEN AVOIDED HAD I PERSISTED IN A VERBAL CONFIRMATION FROM THE CAPT AT THE TIME. THE RESPONSIBILITY IS MINE AND I DID NOT CARRY IT OUT. THERE WERE INSTANCES WHILE I WAS FLYING WHERE I DON'T REMEMBER HIM (AS THE PNF) GETTING A CONFIRMATION FROM ME WHEN SETTING MY ALT; HE WOULD CONFIRM IT HIMSELF. THIS CONFIRMATION DOUBLECHK EXISTS FOR A PURPOSE.