Special type of nginx redirect: redirect only if $request_uri is empty
Ok, I've found the solution. I'm adding this to my existing nginx config:

location = / {
  rewrite ^ http://subdomain.domain.com permanent;
  break;
}


This redirects the top level domain but leaves all the existing "deep" links intact. If anybody wants to know why I'm doing this, it's good for SEO. I'm keeping the old URLs intact (as an archive) but redirect the domain to my new website.