Honestly, you remove about 20% of the fender. However, it's very east, especially with a dremel. Tin snips fo a fine job of cutting the plastic too, though. The main thing you'll have to remove is the structure that supports the seat on the KX. It just gets in the way of the airbox on the KDX. You'll also have to drill 2 new mounting holes in the fender. No biggie there either. I'm no expert bike guru and it only took me half an hour to do the whole thing.