Changing both Products and Billing Plan in a Single Call

 $abill = new AutoBill();
 $abill->setMerchantAutoBillId('vin_test_abill1391560836975'); 
    // This is ID of the AutoBill you want to modify.
  
 $productRemoved = new Product(); // Product we want to remove.
 $productRemoved->setMerchantProductId('monthlySub'); 
    // Simply identify the Product by its ID. No need to fetch it.
  
 $itemRemoved = new AutoBillItem();
 $itemRemoved->setProduct($productRemoved);
  
 $productAdded = new Product(); // Product we want to add
 $productAdded->setMerchantProductId('AnnualSubProduct'); 
    // Simply identify the Product by its ID. No need to fetch it.
  
 $itemAdded = new AutoBillItem();
 $itemAdded->setProduct($productAdded);
 $itemAdded->setMerchantAutoBillItemId('item-425304'); // Unique item ID.
 $itemAdded->setCampaignCode('ANNUALPROMO'); 
    // To apply a Promo to the Product being added.
  
 $modification = new AutoBillItemModification();
 $modification->setRemoveAutoBillItem($itemRemoved);
 $modification->setAddAutoBillItem($itemAdded);
  
 // We want to change to the annual plan.
  
 $newPlan = new BillingPlan();
 $newPlan->setMerchantBillingPlanId('annual-plan-2');
  
 $modifications = [ $modification ];
  
 $abmr = abill.modify(
          true, // Bill prorated period.
          'today', // Make the change effective today.
          $newPlan, // Not changing the Billing Plan.
          $modifications,
          false // No dry run.
          );

 if ($abmr->getReturnCode() == 200) {
    // We got a 200 response code back.
    // The modification may result a net refund or 
    // net charge to the customer.
    $refunds = $abmr->getRefunds();
    $tx = $abmr->getTransaction();
    if ($refunds != null && $refunds->length > 0 ) {
       // Modification resulted into net refund
       // In most cases there should be only one refund
       $the_refund = $refunds[0];
       print('Total refund amount due to modification: '
          . refunds[0]->getAmount()
          . ' . See break down below: ');
  
       if ($tx != null ) {
          printTxDetails($tx);
       }
    }
    else {
       // Refund is null, so there must be net cost to the customer.
       if ($tx != null ) {
          print('Total charge processed during modification: $'
             . $tx->getAmount()
             . ' - see break down below:');
          printTxDetails($tx);
       }
    }
 }