Re: Rechambering from 25-06 to 243???
mtbullet is right.
However, I would also point out that 25-06 is a long action and 243 is generally a short action.
Hence, you may have to replace or block in the magazine to keep the cartridges from sliding around fore and aft and to keep them feeding nicely.
I'd probably be inclined to sell or trade for a short action if I was certain that I wanted to end up with a 243.
What is he wanting to use it for once it's rebarrelled?