Add/change author from PHP

Hi, I have some code to set the author for a post. It works fine when your plugin is not installed or activated, but doesnt work when your plugin is active.

The code is:

$arg = array('ID' => $post->ID,'post_author' => $id,);
wp_update_post( $arg );

Is there a way to set or change the author(s) of a post, from within PHP, that works when your plugin is active?

Thank you.