Actually, I’ve seen this happen both ways. I would disagree with Carl that the neck has no influence on the sound: with an instrument made of wood, the entire instrument resonates to some extent, not just the board. But I very definitely have seen two instruments sent back to two different highly respected shops that returned as much poorer instruments than they went, although nothing was done but the neck, and I’ve seen the opposite happen, too.
I had a 17 which was very very beautiful but imcredibly hard to play, although it was my main instrument for about 5 years. The woman who bought it from me knocked it over putting in the car to go to a gig on a windy day. She had to get a new neck, and the sound was very greatly improved by the change in the neck. As far as I know the neck was all that was done–I saw no evidence of any changes in the body, like the extra bracing L&H sometimes puts in, and she was not charged for anything but a neck.