The 91 KX125/250 triple clamps will swap to the KDX frames, but 92-95 KX125/250 triples will require a stem swap, although those years stem swap may be the exact same press fit diameter as the KDX stem (most '96+ upgrade forks use a steering stem several thousandths of an inch larger, making the KDX stem no longer a press fit, and requiring knurling to increase the o.d. slightly on the stem, + retaining compound.
Well I have now verified that KLX250/300/650 triples through at least 2007 should all be the same clamping diameters as 91-95 KX125/250/500 forks, and I believe from references on this forum, that 91-94 KDX250 triple clamps share these same dimensions. Most 250's use a lighter aluminum steering stem also, for a minor weight savings
So the KLX or KDX250 triples can mount the 91-95 KX125/250/500 forks as a direct bolt-on, and 94-95 are mid valve forks, a bit more advanced progressive damping that helps for higher speed riding and racing.
You'll of course need the KX front wheel as well, as it has a larger diameter 17mm axle which will give you another advantage over the 43mm KLX forks & the stock KDX/KLX 15mm axle in terms of front end rigidity, as well as the external adjustable rebound and compression of KX forks vs KDX/KLX, + the 94-95 having additional compression stacks on the mid valve, for more progressive high speed damping vs previous years with only base valve compression stacks (perfectly fine for casual woods roding, but mid valves help out in hard fast racing use, desert riding, and motocross).
The KX500 triples these years will be a direct bolt-on to the KDX, but 91-95 / 96 / 97-04 KX500 forks and triples are scarce to come by...
96+ KX125/250 use a 20mm front axle, so wheels must come from 90(?) or 91-95.
I hope this helps someone! No stem swap required when going this route!