Client afraid of losing search ranking after website redesign

[watzo99]

My client doesn't want to hurt her search ranking when her redesigned business site goes live.   Her idea to avoid hurting current ranking is to use a 2nd, new domain for redesigned site, run both simultaneously, and when she determines the new one is doing well enough in search, shut the old one down.

I have basic knowledge of SEO and use the basic on-page techniques (on both the old and new site).  I'm pretty sure her idea won't work, and may even hurt the ranking of her original site.  I want to build a case for not going this route.  Can you give me some reasons this idea is not the way to go?

Thanks.