As the new conductors will have lower impedance, some breakers may need to be replaced to interrupt higher fault current. Otherwise, it's likely the only substation equipment needing to upgrade would be series compensation stations which may have lower normal and emergency ratings than the upgraded conductors.
More likely is that lower impedance on the reconductored circuit will cause increased flows on other, non-upgraded circuits, either requiring those to be reconductored, or installing phase-shifting tranformers or reactors to limit current.
More likely is that lower impedance on the reconductored circuit will cause increased flows on other, non-upgraded circuits, either requiring those to be reconductored, or installing phase-shifting tranformers or reactors to limit current.