Yes. Ignoring certain aspects of SEO unique to launching a new version of an existing website can have serious consequences. How serious? It could put you out of business--that’s how serious. For example, I know of an ecommerce company that redesigned its website, which included thousand of product pages, all of which were showing up prominently in Google. Unfortunately, the company doing the redesign of the website had no SEO experience and in creating the new website they changed the URLs without redirecting traffic from the old URLs to the new ones. Suddenly all that traffic from Google was finding nothing but “404 - Page not found” errors, sales collapsed, and within one month the company was bankrupt. Having a 301 redirect plan in place is critical to SEO success when redesigning your website.