I have never really looked into re-boring a rifle barrel. But i feel like the process of boring the entire barrel out to an entirely different caliber and re rifling the barrel is going to be not just labor intensive, but very very specialized labor at that. Its the sort of thing i wouldn't want to trust to anyone but a company that makes their own barrels to do it.
Its going to be expensive, like more expensive than buying a brand new barrel chambered in .257 Roberts and having it installed..
Why not just re-barrel with one as close to stock contour as possible?
You could get a brand new match grade barrel in the same contour and color for probably half the price it will take to get your old barrel bored out.
The barrel maker Lilja might do it, but they are just going to say exactly what I did even if they were willing to.
http://www.riflebarrels.com/