The time to involve an SEO company in your site redesign project is right at the beginning. If you're lucky, you'll work with a full-service outfit that can handle the SEO and design work all at once.
The biggest reasons to include an SEO firm in our site redesign are to save you money and get you results. By waiting until after your site has been redesigned, you risk having your SEO company tell you about all the things your designer did wrong that will inhibit the search engine's ability to rank your site.
Or, you will spend a lot of effort having someone write your site content, only to find out that you need a major rewrite for it to be opitmized properly.
Before you embark on your site redesign project, talk to a qualified SEO expert. Have them review your existing site and talk to them about what you'd like to do with your new site. Finally, ask them what they can do to get your new site on track right from the start for optimal perforance on the search engines.
Read this article on 5 Reasons to Use an SEO Firm for your Website Redesign.