I did just that a couple of years ago. Simply added some wood blocks (1" square pieces I milled from black locust stock I had laying about; tough & very resistant to rot) simply to raise the supplied plastic fenders enough to clear the additional wheel diameter.

If you managed to assemble a 250 by yourself it ought not be a difficult task if you have simple woodworking hand tools available.

Could also be done with square aluminum tube I suppose or even flat stock of sufficient thickness but I think the design approach would have to be a little different.

I've had limited success posting pics here but I'll try again in awhile, see if I can show what I came up with....

All you need to do is add risers to where the fenders mount on the frame. 12" wheels are 4" greater in dia. than 8" but that's overall; from the axle center they're just 2" larger so raise your fender mounts 2" minimum, 2-1/2" might be better yet.

I went with 12" radials as well with the change, they're wider than similar-size bias tires so I had to alter the stand-offs a little so fenders would be centered over new treads, not offset about half an inch.

Use durable material for risers, fasteners of appropriate size, you'll have no trouble.

   I switched my 8" wheels to 12" with radial tires. Etrailer.com had a nice aluminum mag wheel, and good radials. Trailex no longer ships trailers w 8" wheels. They have a 12" fender kit which covers the 12" wheel much better than the 8" fenders. The new fenders are aluminum instead of plastic. I got this kit by calling Trailex. It was about 100 for the kit and 40 for shipping to GA

Thanks for posting that Briancb. I've reached out to Trailex about that 'upgrade'.

I'm entirely satisfied so far with having cobbled something to make the 8" fenders function with the 12" wheels & tires I added a couple of years ago. If the new aluminum fenders are longer, so they cover the front & back of the 12" tires (the plastic 8" ones don't quite) it'll be worth the money... and I do like the idea of aluminum fenders over plastic for durablility. 

   No problem. The new 12" fender kit definitely covers more of the wheel than the 8" fenders. I was worried about rocks flying off the tires, which is why I changed them

For others who may be interested in this upgrade, here's a link to Trailex's website describing what's included in their upgrade kit as well as how it's to be installed.

Current cost (shipping's extra) $180 but that, as with everything else, may change over time.