If you know the ID of the author you can use wp_insert_post specifying the ID and the author ID to it.
$aidi$id = $post->ID; // change this to whathever
$useraidi$user_id = '4'; // change this too
$tehpost$the_post = array();
$tehpost['ID']$the_post['ID'] = $aidi;$id;
$tehpost['post_author']$the_post['post_author'] = $useraidi;$user_id;
wp_insert_post( $tehpost$the_post );
The trick is to specify the ID to update the post. See http://codex.wordpress.org/Function_Reference/wp_insert_postwp_insert_post()
.