I agree with PawPaw. If you're going to change anything with your .25-06, making it an Ackley Improved would make alot more sense than .257 weatherby. Even as an AI you're not gaining much. You will spend alot more buying ammo or loading for the weatherby, and will not gain enough ballistically to ever tell the difference. The .25-06 is already close enough to perfect as it is IMO. Its fun to play around with ideas and possibilities in your head though. If you're in love with the .257 wby just buy another rifle already chambered in it and you will have 2 good quarterbores for about the same money it would have taken to make your '06 into a weatherby.
What kind of rifle is your .25-06? Whats the barrel length and twist rate?
"Most people can't hit a flock of circus tents at those ranges." - P.O. Ackley