I just spoke to a highly regarded gunsmith about trueing and action for me. He told me that once you have modified an action and/or boltface, the same barrel(chamber) will not correspond properly and that making the barrel and chamber fit again is such a tough job that you may as well put a new barrel on and start from scratch.

This doesn't make sense to me.

It is true?

It makes perfect sense. You don't "true" an action by adding material. Rather it is accomplished by taking it away. So, when the reciever threads are made straight the inside of the reciever becomes larger. Think of a simple nut and bolt. If the inside of the nut is enlarged to make it true, then the bolt will understandably have slop when replaced.

so I need to provide a new barrel as well?
What doesn't make sens is that you can not modify the existing barrel to fit.

Depends on the profile. If there is enough shank you can cut the threads off and start over. If it is contoured, you will not have as nice of a fit in the barrel channel, though.

You don't need a new barrel, but Your smith will have to remove the same ammount from the threaded shank of Your barrel as he did from your receiver, to set it back. That makes the chamber too short, so then he'll have to re-headspace the chamber. If the barrel has iron sights, the smith has to take exactly one thread off the barrel and set it back so the sights come back up to center, and then ream the chamber forward the exact same ammont. I have done this kind of work hundreds of times, and it is precise, but not difficult.
If the rifle is a bolt action you'll end up with a little more 'float" in the barrel channel. If it's something else (A lever action for example) You'll have to cut the forend back the same ammount, the magazine tube and so on...

I don't see where the smith would preferr one over the other as there is as much, or more work to rebarreling. To rebarrel he would need to cut threads chamber and headspace and finish the barrel. To set it back the only difference would be timing the thread removal to center the sights if any.

If your gunsmith recut the action threads with a single point,which is usually done when an action is completly trued,then your old barrel will be a loose fit at best when screwed into the action.To reuse your old barrel,the threads would need to be cut off,and a larger thread recut to fit the action threads,if you have enough shank to do this.That's the way i understand it all anyhow.

Express, If the truing up, etc. did not involve recutting the receiver threads to make them concentric with the bolt body bore, and there are no sights, ribs or other encumbrances on the barrel exterior and it is a mauser 98 type, then ,yes, it could be a simple run the barrel in further to take up the slack operation and the caliber marking might disappear under the left side stock line. If ,however, the barrel must be run in a full turn there is one item overlooked so far on this thread.There the matter of picking up the existing chamber dimensions and continuing
that chamber forward. The sensible way is to take a chamber cast before anything is done to determine the expense involved ( and the time) to make the necessary reamer or reamers, or to buy them for one time use. A gunsmith with extensive toolmaking experience can jury rig hand tools and get rid of the step at the end of the case body where someone's reamer continued the chamber under size to the old chamber albeit a reamer above minimum chamber size used for the continuence, A double diameter neck can be reamed to pick up the old neck if the smith has a fleet of neck reamers and so on. This is the real hang up and your smith just may have some bitter experience prompting his answer.