Let’s jut say you’re migrating content from a static site to a Drupal-based site. Let’s say that you don’t want to migrate the content all at once and want to roll out the new Drupal-based site ASAP and migrate content as you’re able to. One thing that you could do is to make your old site static.mysite.com and your new Drupal-based site mysite.com. Then use the Custom Error module and this snippet to automatically redirect to content on the old static site. If the content isn’t in either place, they’ll get a 404 from the webserver of the static site.