Welcome to the Q&A Forum

I think that if you keep the same [seo structure - url, title, h1, content, etc] you should not drop in rankings at all

Check this :

http://www.google.com/support/forum/p/Webmasters/thread?tid=3d2a9df109e09763&hl=en

Conclusion is mirror your site especially with URLS so nobody will know you have a different cms -> you don't loose anything

I suggest that you make the header the same in the main domain and in the blog... So the logo can always link to your home page (shop) and the button Garden Design Blog will always get you on the blog main page. I know it's a bit of work but that's how i see it proper for not confusing search engines or your clients...

Hope this helps

I think it depends on what the redesign consists in. For example a competitor of one of my clients has just redesigned their site but the difference is the old site had more relevancy to a specific keyword where as now the redesign is a general page that doesn't have as much relevancy to the specific keyword.

The result was a drop in rankings. So if you're a simple site with little content or poor structure it's a good thing to redesign... if it's the other way around where you focus more on design and not on content (SEO) i think you will have a drop in rankings