I have a late model KDX220 which basically has the same forks my old '88 KX had --pretty crude by today's standards.
Many KDX owners have retro-fitted newer UpsideDown forks to "upgrade" their suspension. The $64,000 question is --CAN ?? any of the newer [probably KX 125,250- 2000 & later] forks be made as compliant as the old [right-side-up] forks? The bike will never be used for motocross or large jumps --mainly gnarly trail riding in Utah and Colorado. Obviously, there would be less mechanical flexing with the upside down arrangement.
the big upside of upside down forks(npi) is the sprung verse unsprung weight, with upside down, the majority of the forks weight is sprung, and vice versa with right side up. for tight trails, or any riding for that matter i would want as much weight sprung as possible