[jsharf]

When you change a product page, you lose all reviews, but you get a "New Product" link on the old page to link to the new one, giving you exposure.

Amazon can make the "New Product" show up for queries for the older one, but without the stellar reviews. This way modified products still get good exposure and no one will accidentally buy the old one. But there aren't reviews on the same page for a different product. The system can still be gamed similarly, but at least you can't get reviews for the wrong product on the same page.