My experience is that any drop in rankings are extremely brief, especially if you carefully redirect all pages (as the others recommended). There have been occasions that I've seen quite the opposite happen... within 2 days, rankings shot to the top for the bulk of the phrases I was monitoring. Of course, that can be dependent upon your niche.
I have found that the long-term positive of cleaner URLs far outweighs the short-term negative. If you are careful and thorough with your 301 redirects, there might be no negative, IMHO.
