By Drupal upgrade we should understand two aspects: upgrading the core of Drupal or upgrading the modules/themes.
Upgrading Drupal core
Drupal team continuously improves the core by adding new features or fixing bugs.
We need to keep our version to Drupal to the latest version, especially when the latest version includes some security.
In this case we avoid the possibility that someone can exploit the security issues and perform unwanted operation to our site.
Upgrading Drupal 5.x or 6.x to the their latest versions should go smoothly.
Upgrading a 5.x version to a 6.x version could be little bit difficult, because there could be some modules that are working great on 5.x that are not maintained to 6.x.
In this cases, we need to find alternative modules for 6.x that have the same functionality as the old modules in 5.x and integrate them into the system.
In some cases, we will not find those modules and we have to write them ourselves.
Upgrading Drupal module(s)
Not only the core is continuously improved, but also modules are improved to add new features and fix bugs or security issues.
In this case it is also recommended to upgrade the modules to the latest versions.
Upgrading the Theme
A Drupal theme from 5.x will not work for 6.x. In this case we need to upgrade it.
If we are using a custom theme on our 5.x web site, we should upgrade it manually by editing the template files according to the 6.x version.
Contact us for a quote if you need to upgrade your Drupal core, modules or theme.