Redirect all posts permalinks from “.html” to “/”

Add this to your .htaccess below the # END WordPress line:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{HTTP_HOST}
RewriteRule ^(.*)/(.*).html$ news/$2/.html [L,R=301]
</IfModule>

This will redirect domains such as:

http://example.com/category/post-name.html

To the following:

http://example.com/news/post-name/.html