How do I change attachment slug from name to id

Use the add_attachment hook to update the slug after the attachment has been inserted:

function wpse_182454_attachment_id_as_slug( $post_id ) {
    if ( get_post_field( 'post_name', $post_id ) != $post_id ) {
        wp_update_post(
            array(
                'ID' => $post_id,
                'post_name' => ( string ) $post_id,
            )
        );
    }
}

add_action( 'add_attachment', 'wpse_182454_attachment_id_as_slug' );