The vz52 is 7.62 x45, the vz52/57 is not converted to 7.62x39, it was made in 7.62x39. It is a transition piece, being produced just before the vz-58. As all warsaw pact countries were required to equip their forces with firearms that fired the standard cartridge, many existing vz-52 were re-chambered in 7.62x39, but ones marked /57 started life as such.
I have one, and it is a neat gun, that has a completely different recoil impulse to the sks, due to its unique piston design, and vigorously tosses empties forward and left, rather than right. Keep it, you'll be hard pressed to find another. Read the excellent sticky in the red rifles section.