The biggest differences is the 403 is a fully heat moldable while the 400 is not. The 403 is also kangaroo leather which will stretch and mold to your foot over time, while the 400 is microfiber, so it won't stretch much at all. The length and width and volume specs are the same otherwise. I'd only switch if you wanted to save money, but because of the moldable heel and sole, the 403 will likely fit better, assuming you mold them. Plus you already know it fits well. (Note the 403 out of the box has a slightly narrow "bathtub" style sole that is narrower until you mold it.)