One of the things to think about with search engine optimization is that it's a long term thing. If you don't think long-term you run the risk of paralyzing your site because you get too wrapped up in not "risking" a ranking.
That's obviously no good.
Because sites need to change.
In all honesty, when it comes to redesigning, there's a greater risk of impacting your conversion rates than there are of risking your rankings...but then again, you can increase both of them as well.
A few things you need to consider:
1.) Try to keep the same URLs. This is the biggest thing that can impact your rankings...having URLs that used to rank but that are suddenly non-existant. If you have to change your URLs, make use of 301 redirects to point the search engines toward the new pages.
2.) Make sure that the changes are still search friendly. Don't change to an all Flash site or to a Framed site or something else that might cause problems for the engines.
3.) Realize that you WILL see an impact on your rankings, maybe for worse, maybe for better. But, with the redesign, you should also be thinking broader. Don't worry just about where you rank for one phrase, worry about where you are GOING to rank for tons of phrases.
There's an article over at Search Enigne Watch that talks about the "Retaining Traffic After a Web Site Redesign" that's worth a read...
I guess my best advice is to not let fear control the need for change.