class HprTest extends Plugin{
public static function getSubscribedEvents(){
return [
'Enlight_Bootstrap_AfterInitResource_hpr_legacy.the_service' => 'decorateService',
];
}

public function decorateService()
{
$coreService = Shopware()->Container()->get('hpr_legacy.the_service');
$refl = new \ReflectionClass(get_class($coreService));

$field = $refl->getProperty('config');
$field->setAccessible(true);
$config = $field->getValue($coreService);

$service = new TheNewService($config);
Shopware()->Container()->set('hpr_legacy.the_service', $service;
}
}