The search engines (like google, yahoo, bing) put a lot of stock in how long your website and domain have been around. While you can purchase a new domain and redirect your old one to the new one, your best bet is to use your existing domain/website if at all possible. If you're redesigning or starting from scratch and you have to use a brand-new domain for some reason, you can expect at least somewhat of a loss in search engine traffic. It could be anywhere from a few weeks to a few months or more.