The municipality of Brisbane (USA - California) needed to migrate their website from Drupal 6 to Drupal 7.

As any administration website the needs are really specifics and must answer to those critria :

• no visual change should be visible to the end user,
• backoffice changes must be avoided as much as possible (except if requested),
• the website should never be offline during the migration,
• the security of the server and the website is a prerequisites.

Basically one of the key of working with administrations is that your work must be transparent for the end user (and as much as you can for them).

Migrating their website mostly consisted in making a custom module written in PHP to reput every data at its right place in the new drupal 7 database structure. I have written an article about that and you can consult it here.

The funny part of it is, that the work last one month for at the end running a script which last 3 min :)

Well, Brisbane has now a brand new drupal 7 website and that's what matter.