I just had a friend send me some good info regarding my question:
I don't think the company modified the older fleet, they just came that way. I was told by one of our Check Airmen that the newer design leading edge cuff is a vortex generator that delays airflow separation along the wing/fuselage juncture. Airflow separation means drag, so it is a drag reduction device, which means they must have discovered something in the way of an easy fix to reduce drag. My dad used to always say that the intersection of the wing and fuselage was one of the most difficult areas to design. If you get it wrong, the drag penalty can be huge for long range type aircraft. My dad was a aerodynamic flight controls design specialist on the XB-70 and the B1 among others - two long range aircraft that were designed before computer aided design was available. Amazing what those guys could do with a slide-rule.