Preserve Search engine index while shifting to new Domain

If you use the Redirection plugin, you can just setup a regex redirect from (www.)?olddomain.com to www.newdomain.com (you should be using www., it’s better for SEO as it helps page load time. and that single rule should pretty much do it for you.

You can do this same thing in .htaccess, and it would be more efficient, but I couldn’t tell if you had the capability for that or not, rewrite rules as follows should get you most of the way there (assuming you have the capability):

Options +FollowSymLinks 
RewriteEngine on
RewriteRule (.*)$ http://www.newdomain.com/$1 [L,R=301]