Change the content type of a many nodes at once, without losing data from existing fields. Requires the Node Convert, Views, and Views Bulk Operations modules.

First, install and configure the Node Convert, Views, and Views Bulk Operations modules.

Next, follow these steps:

1. When using Drupal 6, go to "admin/build/node_convert_templates". When using Drupal 7, go to "admin/structure/node_convert_templates".
2. Create a new "Node Convert" Template, following the the provided steps (and also check "Create Action").
3. Create a new view with the options you require.
4. Add Views Bulk Operations under fields.
5. Configure all options of Bulk Operations as necessary.
6. Select "Convert a node" as an operation or an action that was created together with the conversion template.
   
   Note that VBO Operations are gathered from two sources:
   • Drupal core actions (hook_action_info() and advanced actions added through the Actions UI)
   • Rules 2
7. Save the view. View it.
8. Select the necessary nodes and click the "Convert x" button.