We're in the process of migrating sites between Drupal 6 and D7 and that's also involved some theming work. One of the changes in our theme is the main site title has been changed from H1 to a div, the page title from H2 to H1, and content headings from H3 to H2. This means any H3's in the old site now need to be H2's, H4 -> H3, and so on.
Luckily, we have regex to the rescue. All I need to do is find:
and replace with
either in the import process or using the handy Search and Replace Scanner module, then rinse and repeat with and further headings.