Can anyone help me sift through some conflicting advice? I've been tasked with launching an updated site for a medical clinic that, through the work of a local SEO consultant, has a couple of #1 and #2 rankings for critical search terms in Google (with lesser rankings on Yahoo). 

The site we'd hoped to replace is antiquated visually and is difficult to navigate.  The proposed site is more SE friendly, has a content management system built with the same in mind, and will have the same domain and title pages.  About 20% of the textual content will change on the home page.

Our SEO consultant has warned us against making this change. His view: you probably won't lose anything, but you might, and if you do I might not be able to ever get your rankings back.  Moreover, I can't guarantee anything. The website owner's view: not worth the risk.

Neither of us is tech professionals (obviously) but something seems glaringly odd here.  If you can't update or launch a new site without breaking your rankings, why isn't every site out there circa 2002? (Ours is circa 1998).  How does anyone ever replace an out-of-date site?