public function isAuthorValid(ExecutionContextInterface $context)
{
    $original = ... ; // get old values
    if( $this->status !== $original->status && $this->status === 'Racked' && $original->status === 'Unracked' )
    {
        // check ftp and ssh connection
        // $context->addViolationAt('status', 'Unable to connect etc etc');
    }
}

namespace VendorYourBundleValidationConstraints;

use SymfonyComponentDependencyInjectionContainerInterface;
use SymfonyComponentValidatorConstraint;
use SymfonyComponentValidatorConstraintValidator;

class StatusValidator extends ConstraintValidator
{
    protected $container;

    public function __construct(ContainerInterface $container)
    {
        $this->container = $container;
    }

    public function validate($status, Constraint $constraint)
    {

        $em = $this->container->get('doctrine')->getEntityManager('default');

        $previousStatus = $em->getRepository('YourBundle:Status')->findOneBy(array('id' => $status->getId()));

        // ... do something with the previous status here

        if ( $previousStatus->getValue() != $status->getValue() ) {
            $this->context->addViolationAt('whatever', $constraint->message, array(), null);
        }
    }

    public function getTargets()
    {
        return self::CLASS_CONSTRAINT;
    }

    public function validatedBy()
    {
       return 'previous_value';
    }
}

services:
    validator.previous_value:
        class: VendorYourBundleValidationConstraintsStatusValidator

        # example! better inject only the services you need ...
        # i.e. ... @doctrine.orm.entity_manager

        arguments: [ @service_container ]
        tags:
            - { name: validator.constraint_validator, alias: previous_value }

use VendorYourBundleValidationConstraints as MyValidation;

/**
 * @MyValidationStatusValidator
 */
class Status
{

// src/Acme/AcmeBundle/Validator/Constraints/IncrementOnly.php;
<?php
namespace AcmeAcmeBundleValidatorConstraints;

use SymfonyComponentValidatorConstraint;

/**
 * @Annotation
 */
class IncrementOnly extends Constraint
{
  public $message = 'The new value %new% is least than the old %old%';

  public function getTargets()
  {
    return self::CLASS_CONSTRAINT;
  }

  public function validatedBy()
  {
    return 'increment_only';
  }
}

// src/Acme/AcmeBundle/Validator/Constraints/IncrementOnlyValidator.php
<?php
namespace AcmeAcmeBundleValidatorConstraints;

use SymfonyComponentValidatorConstraint;
use SymfonyComponentValidatorConstraintValidator;

use DoctrineORMEntityManager;

class IncrementOnlyValidator extends ConstraintValidator
{
  protected $em;

  public function __construct(EntityManager $em)
  {
    $this->em = $em;
  }

  public function validate($object, Constraint $constraint)
  {
    $new_value = $object->getIntegerField();

    $old_data = $this->em
      ->getUnitOfWork()
      ->getOriginalEntityData($object);

    // $old_data is empty if we create a new NoDecreasingInteger object.
    if (is_array($old_data) and !empty($old_data))
      {
        $old_value = $old_data['integerField'];

        if ($new_value < $old_value)
          {
            $this->context->buildViolation($constraint->message)
              ->setParameter("%new%", $new_value)
              ->setParameter('%old%', $old_value)
              ->addViolation();
          }
      }
  }
}

// src/Acme/AcmeBundle/Resources/config/validator.yml
AcmeAcmeBundleEntityNoDecreasingInteger:
  constraints:
     - AcmeAcmeBundleValidatorConstraintsIncrementOnly: ~

// src/Acme/AcmeBundle/Resources/config/services.yml
services:
   validator.increment_only:
        class: AcmeAcmeBundleValidatorConstraintsIncrementOnlyValidator
        arguments: ["@doctrine.orm.entity_manager"]
        tags:
            - { name: validator.constraint_validator, alias: increment_only }