I recently copied a client’s WordPress website from one domain to another. In order to retain SEO for the old website, I needed to redirect every page on the old site to the same file on the new site.
In order to do that, I added the line
RewriteRule ^(.*)$ http://newsite.com/$1 [R=301,L]
directly after the code
# BEGIN WordPress
in the old site’s .htaccess file. It works great immediately.