How to switch subdomain with root domain and redirect traffic?

I want to move subdomain to root, and create a new root. I have root.com. I want to move it to sub.root.com and install a new Wordpress intall at root.com. Any idea how I redirect the traffic from root to sub.root without affecting the new site? Or do I just have to move, wait for the search engines to redirect and then install the new copy on root.com?