I am using a custom payment and passing in the ImplementationClass when adding the payment to the cart. Using cart.AddPayment method. However, when ProcessPayment is called in my custom payment gateway I get the IPayment Object which is iternally a 'SerializablePayment' object.

Following last comment from Khiem, I understand that I should get a CustomPayment object back. Is there a something I need to do in ProcessPayment method to convert?

    [JsonConverter(typeof(PaymentConverter))]
    public class BarclaysFusePayment : SerializablePayment
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="BarclaysFusePayment"/> class.
        /// </summary>
        public BarclaysFusePayment()
        {
            this.Properties = new Hashtable();
            this.BillingAddress = (IOrderAddress)new SerializableOrderAddress();
            this.PaymentType = PaymentType.Other;
            this.ImplementationClass = typeof(BarclaysFusePayment).AssemblyQualifiedName;
        }
        ...
    }

The following code use to work with Mediachase version but now thows an exception as it cant cast 

public class BarclaysFusePaymentGateway : AbstractWexPaymentGateway, IPaymentPlugin
{
        public PaymentProcessingResult ProcessPayment(IOrderGroup orderGroup, IPayment payment)
        {
            var cardPayment = payment as BarclaysFusePayment;
            var paymentSuccessful = false;

           //payment processing logic

           if (paymentSuccessful)
          {
               return PaymentProcessingResult.CreateSuccessfulResult(string.Empty);
           }
           else
           {
               return PaymentProcessingResult.CreateUnsuccessfulResult("Error processing payment");
           }
        }
}

Admittedly, we've not upgraded in a few years we are upgrading from Mediachase.Commerce 11.8.3.0 to Mediachase.Commerce 13.32.0.0