If you don't give search engines exact instructions of where to find content as it moves (via 301 redirects), the engine has no way of carrying over the link metrics that point to the original URL to the new one. Because link metrics are such an important part of search rankings, the consequences of not having a solid SEO plan for the change can lead to lost rankings and traffic as well as frustrated users who had previously bookmarked the old location of the content or try to find it through existing links. Therefore, it’s important to set up 301 permanent redirects from old pages to new ones in order to help search engines and users understand where the content has moved to. Remember, if even a single character in a URL changes, including capital letters to lowercase, the content has moved and most likely the engines won’t be able to correctly transfer the link metrics. Get the full story at Search Engine Watch