I don't think it will be an easy swap. I believe the rotor mounting pattern changed, and the rotor is a different size too. Neither the 250 rotor or the 400 rotor would work with both the wheel and the caliper/mount, so you'd have to do some modding and/or get custom parts.
I think the 300 wheels are a closer fit, possibly even a direct swap. However, the 300 switched to an electronic speedo, so the 250's cable-driven speedo wouldn't work with it.
The NewGen 250 and Gen2 500 wheels are almost identical for mounting. I actually put
aftermarket 250 wheels on my 500, with only a few little OEM spacers and such needing to be swapped. The 500 wheels' appearance is a bit dated (since they're from the mid-'90s), but they should be functional, and the look would be unique if nothing else.
https://www.ex-500.com/threads/40125/ is the post on the 500 forum, which has more details about fitting the 250 wheels to the 500. If you want to go that route, that thread should give you an idea what 500 parts you'd need to swap along with the wheels themselves.