Yup, you'll definiytly need to respring it at both ends. For trail riding, you may, or may not want to smooth out the 2 stroke hit, i.e. make more tractable, you can add a flywheel weight 11, or 13oz would be best, also you can retard the timing about 4degrees (easy to do), and for more low end, you could add a Gnarly pipe. Oh, and keep an eye on the shock linkage, as the needle bearings need to be greased often, gas from the overflow washes it away. I routed my overflow tube away from that area.
I also have a 99 kx250 and love it.
