The server needed an upgrade … so the migration to the next Debian release had to be done. And of course the migration of the CMS.
This was more complicated than expected … I wasn't able to find a smooth migration path from Drupal 6 to 7 for me (some important modules weren't available for Drupal 7 anymore). So after much hassle I decided to go with WordPress.
I googled around and found some helpful sites and a SQL script which I modified to suit my needs. Maybe this can be helpful for somebody else:
The SQL script is attached below. It's a adopted version of the script which can be found here. My main change is an added function to create URIs for the posts based on the blog post title.
Please note: don't use the script as is but change it to fit for your concrete migration. Use it at your own risk!