http://www.xt660.com/showthread.php?t=7560
This was the one I originally posted but as I said earlier I don't think anyone has done it.
Considerations have got to be about the fork brace and fender removal, the shorter fork movement, the position and mounting of the brake caliper and disc at the front end. No problem is insurmountable but it may be expensive if it needed new R forks and headstock because of the head angle/trail to allow the 21" enough clearance from the downpipes etc.
At the rear it would seem to be a simple replacement of wheel and spacers for the narrower R wheel and perhaps attention to the suspension links (bones) because of the altered ride height at the front.