While the tripple clamp will swap from the KX250 to the KX500, the stem on the KX250 is larger than the KX500. I performed this swap on a 1999 KX250/KX500, and solved the difference in stem size with a small piece of shim stock wrapped around the stem. I raced the bike for several years, and then went to the 2002 KX500, where I just stayed with the KX500 forks and had them tuned by Acme.
In a nut shell, the swap works as long as you are willing to address the different size stems.
Rick