Last night I migrated the oostpoort website from Drupal 5.x to Drupal 6.x. The following steps did i executed to successful migrate the website.
1. Downloaded for each used 5.x module the 6.x release.
2. Make first backup.
3. Upgrade Drupal 5.20 to latest available level 5.22.
4. Upgrade all used Drupal 5.x modules to latest available level.
5. Make second backup.
6. Disable all none core related modules
7. Uninstall update_status 5.x module
8. Remove all Drupal 5.x modules from file system
9. Remove Drupal 5.x core files from file system.
10. Install Drupal 6.x core on file system
11. Configure settings.php (Take the new 6.x template as base) with correct database settings. Set $update_free_access to TRUE.
12. Access the website.
13. Run update.php two times
14. Now the system runs without any issues on Drupal 6.x
15. Install all needed Drupal 6.x modules.
16. Enable all modules and run again update.php
17. Change $update_free_access to FALSE in settings.php
18. Optional: Update theme template
19. Optional: Make third backup.
– All these steps took four hours in total.
– If in the drupal configuration the start URL is “frontpage” replace it which “node” else the frontpage is not working anymore.
– If any of these steps fail restore the website which the available backup.