Forum Moderators: phranque
The best way to preserve your traffic (and PR) is to set up 301 redirects on all the old urls, to point them to the new content. That way, search engines will understand what has happened, and eventually index the new site structure.
Most people, though, won't notice and will never update the bookmarks and links, so you may have to keep those redirects in place for a long time.
You could also just create a custom 404 error page telling that your site structure has changed, and please start at your home page. But that may irritate users.
If you depend on having lots of your pages indexed in SE's, be aware that they might not index dynamic pages as well as static pages, depending on the url format.
If you have a shopping cart, or user accounts, you may want to make sure that they're migrated to the new server as well, so that people don't lose their data.