Upgrading - more sophisticated approach

 (http://drupal.org/node/323089)

Convert a Drupal 5 theme to 6

(http://wtanaka.com/drupal/convert-theme-6)

Upgrading Drupal

( http://drupal.org/upgrade/ )

Converting 5.x themes to 6.x

( http://drupal.org/node/132442 )

Converting 5.x modules to 6.x

( http://drupal.org/node/114774 )