When upgrading your CiviCRM site from 1.x to 2.x you might have to upgrade some custom modules. If you're upgrading to CiviCRM 2.1 or higher, this also requires that you're running Drupal 6. Upgrading your custom Drupal 5 modules for Drupal 6 is pretty easy: you simply download the
coder module which will point out almost all required changes to your module with direct links to the documentation page about
converting 5.x modules to 6.x.
Upgrading your custom CiviCRM modules to be compatible with 2.x however won't be as easy since a bunch of functions have been replaced or removed. The idea is to motivate developers to use the internal API's directly and not use these helper functions anymore. There is a developer discussion about the
API Migration from 1.x to 2.x and while it offers some indication as to what functions were replaced, it doesn't provide a valid alternative for all removed functions.
