For a client running OpenCart v. 1.5.1.3.1 I've just made the following changes to their WorldPay controller, so as to call to a different WorldPay URL.
In catalog/controller/payment/worldpay.php on around line 12
$this->data['action'] = 'https://select.worldpay.com/wcc/purchase';
became...
$this->data['action'] = 'https://secure.worldpay.com/wcc/purchase';
Meanwhile I've checked a client's site running OpenCart v.1.5.4.1 and the same area of code already looks like:
if (!$this->config->get('worldpay_test')){ $this->data['action'] = 'https://secure.worldpay.com/wcc/purchase'; }else{ $this->data['action'] = 'https://secure-test.worldpay.com/wcc/purchase'; }
Hope that helps anyone who missed the email and is wondering why their WorldPay payments have stopped working after the 30th May!