Drupal 5.x to 6.x
Irgendwann steht die Migration einiger Drupal 5.x Seiten nach 6.x wohl an. In dem Zusammenhang gibt es einen Artikel auf Groups.Drupal: PHP script for the migration of content from Drupal 5.x to 6.x.
The only solution in cases where an upgrade is not possible, then, is the use of automated migration scripts. Currently, there is no such unified migration script available. I have broken down the script I used to migrate my 5.x site to 6.x for the migration of users, simple-content-type posts, comments, taxonomy, & forum posts. Migrating the theme itself (blocks, nodes, templates, etc.) would require one to get their hands dirty with PHP.