If I wanted to change a possible shot out barrel on a BRNO ZKK601, and found a suitable donor rifle or barrel only, is it just a case of getting a gunsmith to unscrew shot out barrel and replace with the new one (and reproof?)

What are the legal implications in terms of buying the donor rifle? Is that good reason to have two 308s on my ticket? Am I allowed to purchase just a barrel from somebody or must I be an RFD?

Many thanks