Retain as much of our current SEO rankings as possible

Hello we are about to launch a new version of our websites, some pages (especially home pages) will have urls that already exist and page updated, other areas of the site will have new urls but have old pages that are similar - so would we redirect them? - Is there a link you can point me to that gives the best protocol in this situation so we retain as much of our current SEO rankings as possible - many thanks in advance for your help - Nicky