VHOSTs all done
That's it. The last website has been moved to Drupal 6.
This morning, I took a mysqldump backup of the last backlevel public site (migrated to Drupal 5, yesterday), and created a new "internal" site from it. Using the same magic incantations as before (log on, disable modules, change httpd vhosts, update.php, enable modules), I managed to get this last site running properly on Drupal 6, internally.
Once I was satisfied that it worked as expected (I had one module to upgrade, and one left to re-enable), I switched the vhost for the external site to point to the new Drupal 6 configuration. A quick httpd restart and it is done.
I'll take my time re-enabling the remaining one module, and exercise the site a bit; there's liable to be some sort of minor problems to be fixed, but there's no going back now.
I'm done!