The change to the bolsters started in the late 70's when Case stopped being as careful in the final hafting. so the bolsters started becoming somewhat blockier.
However, the big change occurred in 1985/86 when Case started stamping bolsters for all knives out of flat sheets of nickel silver. The traditional way prior to that point had been to die-form the bolsters out of square nickel silver bars, so the bolsters as formed were half-round to begin with. During final hafting of the knife, the handles were hafted to match the half-round bolster shape.
After 1985 with the bolsters stamped from flat stock, the only rounding that occurs happens during the final hafting, which seems to vary from pattern to pattern. You will see more rounding on the modern "pocket worn" knives.