So, I’m finally going to take the plunge; first with a test, local site, and then with mattbirgit.de.

Only found one point to watch out for, in addition to the clear advice at: http://drupal.org/node/570162: It really is important to update to the current Drupal 6 before upgrading; that isn’t just ‘in case’ some assumptions are not fulfilled; the update script refuses to even start without it.