Ernesto

The Brno ZKK601 is a short length action and was chambered in .222, .223, .243, .308 amongst others.

The ZKK600 is the medium length action for 7x57mm, 7x64mm, .270, .30-06, 9,3x62mm etc.

You won't be able to rechamber a ZKK601 for the longer .30-06 or 9,3x62mm cartridge as it won't fit through the action. Also the magazine box and feed rails will need to have different profiles in order to present each cartridge correctly to the Mauser style extractor.

Have you fired the ZKK .308 to check accuracy?
You could have the gunsmith screw in the ZKK601 barrel one complete turn and rechamber it in .308 Win.
Post-'68 ZKK's have notoriously slack chambers and if the barrel is still in good condition rechambering will increase accuracy and lead to longer brass life.

I have a ZKK601 in .308 and it is a light and handy rifle chambered for a relatively powerful cartridge. Factory .308 ammo is pretty cheap and comes in a wide variety of bullet weights and types.

I'd leave the 9,3x62mm as-is and buy another CZ550 in your calibre of choice... much cheaper and better resale value if required later on.

Cheers

TM