Looking through the code there is a very relevant example:
function woocommerce_paying_customer( $order_id ) {
$order = new WC_Order( $order_id );
if ( $order->user_id > 0 ) {
$old_spent = absint( get_user_meta( $order->user_id, '_money_spent', true ) );
update_user_meta( $order->user_id, '_money_spent', $old_spent + $order->order_total );
$old_count = absint( get_user_meta( $order->user_id, '_order_count', true ) );
update_user_meta( $order->user_id, '_order_count', $old_count + 1 );
}
}
add_action( 'woocommerce_order_status_completed', 'woocommerce_paying_customer' );
which reminds us that the $order_is passed to the woocommerce_order_status_completed
hook. From the $order_id, you can create a new order object, with the user_id as a property.
Armed with that knowledge, I think we can just fix the inner guts of the function get a new user object from that user ID, and remove the old role and finally apply the new role.
function wpa_120656_convert_paying_customer( $order_id ) {
$order = new WC_Order( $order_id );
if ( $order->user_id > 0 ) {
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( 'subscriber' );
}
}
add_action( 'woocommerce_order_status_completed', 'wpa_120656_convert_paying_customer' );