Keeping PR and SERP with New Site Built?

I'm wondering if there is a way to keep your domains Page Rank and location in the Google search results (within reason) if you take a 6 year old domain name, that has always had the same products listed, and:
1) change hosting provider
2) build a completely new Search Engine Optimized site with shopping cart
3) keeping the exact same products listed...just in a better layout
4) the name of the site is the companies name
5) international manufacturers site so not for just one town/state/country

I ask because I have a client who currently has some great first page rankings, but has decided to build a completely new site with a website company who will host the site on their servers. The only thing that will be the same will be the domain name. The whole site structure and shopping cart will be completely different, so I know the links pointing to sub-pages will be lost, but what happens to the links that are pointing to their main URL and is there a best way to handle this change?

Thank you all in advance for you thoughts and consideration.

You will need to create 301 redirects to map the old URLs to the new structure. May still lose some search engine placement, but this will help a lot and cut down on it.

Yes, 301 redirects will help you a lot. if possible try to use the same file names in the script you will use (don 't need to redirect).