What version of Ken's? He's released many packs over the years, I think I have about six different ones on my A2K, although only the latest ones are the same visual style (4.x). I think over the years I've downloaded just about every icon collection on Aminet.
If you're not sure if it's that particular set of icons, try downloading something else?
Like I said, I don't use a tool, I just right-click and get Information, then drag & drop the new icon image right on top of the old image, to change it. Should work fine assuming you have 3.9 and at least a couple of the Boing Bag updates, Peter K. icon.library, etc...
