WordPress MU

I'm planning to move a WordPress.org website to WordPress MU for scaling and future planning purposes.

The current permalink structure is http://www.website.com/post-name/

When I move it to a WordPress MU installation the permalink structure will be http://website.com/a-site-name/post-name/

I’ve got about 400 posts I need to have a 301 redirect on. The only way I know how to do this is by manually creating 400 301 redirects to redirect every post. That would be 400 lines of code in my .htaccess file!

Is there an easier and more efficient way to do this?

There's a redirect plugin that might help. I dunno. It's called dean's Permalink migrate something something. Google it and see if it can work for you.