The correct way is to use the 'type' property', not the 'old_type' property. This is how form_alter() callbacks in core (for HEAD) alter the node type settings form - e.g. see upload_form_alter().
That's true for the most part but not for the conidtion:- ('node_type_form' == $form_id) is TRUE. In that case, $form['type'] isn't set. I believe that's why the upgraders handbook is talking about 'old_type' for some reason. regards, --AjK
I didn't write the "Changes to node type settings form" stuff (although I did write the rest of <http://drupal.org/node/64279#node-type-settings>), so I don't know what the reason is for using 'old_type' in the example here. Can someone explain to me why this example uses 'node_type'? If not, then I will modify this page so that it matches the examples that can be found in core.