do you have to change the bolt or action for a new caliber?

If I were starting such a project, I would start with a .30-06 action for several reasons. The first being is that the case head of both cartridges are identical and the bolt would not require work. The Magazine could be replaced with a .308 length magazine and follower. I would prefer to have the original magazine and follower reworked to shortened, but left longer than the .308 standard to be able to seat the 150gr, 165gr and 180gr bullets with the base of the seated bullets to be located at the junction of the neck and shoulder of the case. The main reason the .308 falls so far behind the '06 with the heavier than 150gr bullets is because so much of the bullet takes up room that the powder should. In fact I'd look long and hard at eliminating the 180gr bullets and staying just with the 150gr and 165gr bullets. I'd stick with the Nosler Accubond bullets. if you furnish the gunsmith with about five dummy rounds, the gunsmith should be able to throat the barrel to those dimensions. With a 1/12 twist in a match grade barrel, I like a Douglas, you should get a tack driver. As the final step, I'd question the gunsmith on his views on cryogenics and have the barrel and action plus bolt sent off to have it done. What ever you do, be sure you get exactly what you want. You're the guy paying for it and you're the one who should be happy with it.