Make sure you or your designer knows something about SEO before you re-design your site. In a lot of cases it will actually help your rankings to get a good developer to throw an SEO-friendly site up. Unfortunately, most of the time you'll just get scammed by people who say that they're knowledgeable and good at SEO.
Keep in mind that just because a site looks good doesn't mean that everything is fine under the hood. Also, if the designer does anything that looks like it was done for SEO (lists your keywords everywhere, spams them in title/description tag, etc.), he or she probably has no idea what they're doing. Good SEO doesn't look like SEO.
There's more, but the simple answer is: If you hire someone who knows what they're doing, you stand a good chance of actually moving UP in the SERPs by changing your design.