3 issues are getting muddled up here.

1. Bellying, ie the top has pulled up (usually behind the bridge, there may be sinkage in front of the bridge). A bridge Doctor will fix this, and IMHO change the sound less than re-strutting the top (as described above).

2. Neck angle. This is the angle of the neck joint. For some reason old Yamahas are particularly susceptible to the joint creeping.

3. Neck progression (curvature). Necks are not supposed to be straight, but to arch forward very slightly so that the strings leave the neck at the same angle wherever you fret it. This is controlled by the truss rod if there is one, the steel bar if you have a reinforced non-adjustable neck (Martin went through a period of this), and prayer and fasting if you have a non-reinforced neck.