感谢所有帮助我找到答案的人（0个帮助者）！我自己找到了解决办法。

function wpa_120656_convert_paying_customer( $order_id ) {

    $order = new WC_Order( $order_id );

    $user_id = $order->user_id;

    $customer_orders = get_posts( array(
        \'numberposts\' => -1,
        \'meta_key\'    => \'_customer_user\',
        \'meta_value\'  => $user_id,
        \'post_type\'   => \'shop_order\',
    ) );

    if ( $customer_orders > 1 ) {
        update_user_meta( $order->user_id, \'paying_customer\', 1 );
        $user = new WP_User( $order->user_id );

        // Remove role
        $user->remove_role( \'customer\' ); 

        // Add role
        $user->add_role( \'dovclient\' );
    }
}
add_action( \'woocommerce_order_status_completed\', \'wpa_120656_convert_paying_customer\' );