I had a potential client approach me the othe day abotu updating their website and performing a full SEO to their site. They stressed that they wanted to change the page urls as well.
They had the idea right, they did need to adjust page urls, but what I needed to point out to them is that when the search engines crawled the site they would rank the new pages as they were and their page rank could actually go down. As well when they changed the page urls, they would create a situation where any text link they had already established would encounter a 404 error, as that page would be gone. There are some thing that can be done to help this.
On is to create redirects for the pages that you are changing. This means that your established links will continue to work. Also make sure to adjust your 404 error message page to create an easy re-entry to your site. If someone gets to the 404 page you definitely want to make sure they can get back to your site, not get frustrated and go somewhere else.
Be careful, additionally, to avoid orphaned pages. Search engines consider sites with orphan pages to be unprofessional, and not worthy of getting a high rank. This kind of website is under construction or is the result of a poor design process. If your pages aren’t linked properly each engines won’t index them and will consider them irrelevant for the search.