On one of my sites, funlol.com, I have installed a new CMS. The problem with this is that I can't keep my old URLs (it's complicated). I have some very old, high ranking pages that I really can't afford to lose. I have two options 1) 301 redirect to the new pages 2) Having an old URL and a new URL. i.e. www.funlol.com/19393/britney-spears-nude.html and a www.funlol.com/funpages/britneyspears.html (might be duplicate content penalties) What would you do to minimize loss of search engine traffic?
Go with the 301 redirect. It'll take a while to point all those old links to their new pages, but it'll be worth it.