(1) Keep the domain as it is for the new ecommerce. Put the ecommerce on site.com.
(2) Move the blog from site.com to site.com/blog
(3) Redirect 301 the old links to the new links.
(4) If you move something, you will likely lose about 30% of link value, even if you do redirection. After a year, you will lose around 40% of the current link value.
- Domain Migrations: Lessons from the Moz Transition – Webinar – Moz
- Redirection – SEO Best Practices – Moz
- Google’s Matt Cutts on Redirects, Trust + More – Whiteboard Friday – Moz
- Web Site Migration Guide – Tips For SEOs – Moz
- SEO Guide: How to Properly Move Domains – Moz
- Domain Migrations: Surviving the „Perfect Storm” of Site Changes – Moz