The accepted SEO practice is to use hyphens over slashes. When you have something like .../category1/category2/my_product, the categories are treated as levels by the search engines. In years past, it was considered harmful if there were more than three levels. But if you have .../category1-category2-my_product, it is considered one level and the keywords used in category1 and category2 apply directly to the product page. Also, in the SEO 5 addon, which is another url rewriter, one of the url options is to create a url like .../product/product_name, where the first word is actually product. The result is that the sites using that format rank for the keyword product, which is a total waste. So I don't recommend making this change. If you are determined to do it though, it can be done so good luck.