How can I alter an existent CCK field? I have a "first name" field, and I would like to programatically change its attributes. I want to modify this field, but not to create one new field again.  

Is it possible?
In this resource http://drupal.org/node/726282 you will find a very useful description about CCK fields and his relation with the function to alter forms, called hook_form_alter(). in few words, you must also use an after_build() function.