Question: I have two roles.

A: The user sends a post for moderation

B: The user publishes posts without moderation 

For new users the role of A. 
How do I do if I approve of his first post, the role is changed to B?

Answer: 
You can attach code to change role when post status is changing from "pending" to "published" see documentation: <http://codex.wordpress.org/Post_Status_Transitions> Then, if user has role "contributor" you can change this to "author":

```
    add_action( 'pending_to_publish'. 'my_function', 10, 1 );
    function my_function( $post )
    {
        $wp_user_object = new WP_User( $post->post_author );
        if ( in_array('contributor', $wp_user_object->roles ) ) {
            $wp_user_object->remove_role( 'contributor' );
            $wp_user_object->add_role( 'author' );
        }
    }

```
From the [Codex Post\_Status\_Transitions](http://codex.wordpress.org/Post_Status_Transitions) you have hooks on the post status change:

```
function on_publish_pending_post( $post ) {
    // A function to perform when a pending post is published.
}
add_action(  'pending_to_publish',  'on_publish_pending_post', 10, 1 );

```

You have access to the $post variable.
So you have to pull the author id, and change his role...

Cheers