The link in Ian's post shows you how to keep your existing pages and use them as redirects. The advantage to this is that search engines will continue to spider them and you also won't get any 404 errors if anyone arrives from an existing search result.
Example:
http://www.audiotoolers.com/vinyl.php where vinyl.php is just a redirect.
( I was going to link to that info but didn't know if it was allowed
)
