Changing Products for an AutoBill

 $abill = new AutoBill();
 $abill->setMerchantAutoBillId('vin_test_abill1391561675069'); 
    // This is ID of the AutoBill you want to modify.
  
 $productRemoved = new Product(); // Product we want to remove.
 $productRemoved->setMerchantProductId('regular-avataxed-product'); 
    // Identify the Product by its ID. There is no need to fetch it.
  
 $itemRemoved = new AutoBillItem();
 $itemRemoved->setProduct($productRemoved);
  
 $productAdded = new Product(); // Product we want to add.
 $productAdded->setMerchantProductId('avataxed-ott-product');
    // Identify the Product by its ID. There is no need to fetch it.
  
 $itemAdded = new AutoBillItem();
 $itemAdded->setProduct($productAdded);
 $itemAdded->setMerchantAutoBillItemId('item-987654'); // Unique item ID.
 $itemAdded->setCampaignCode('OTT'); 
    // To apply a promo to the Product being added.
  
 $modification = new AutoBillItemModification();
 $modification->setRemoveAutoBillItem($itemRemoved);
 $modification->setAddAutoBillItem($itemAdded);
  
 $modifications = [ $modification ];
  
 $abmr = abill.modify(
          true, // bill prorated period
          'today', // make the change effective today
          null, // 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: ');
  
       // To give the customer a break down of how we reached 
       // the refund amount, we must parse the $0 transaction 
       // that accompanies this.
       if ($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);
       }
    }
 }