Override a virtual method in a partial class

I am currently working with the nopCommerce source code and trying my best to avoid editing the source at all, but instead using partial classes and plugins that are separate from the source code, should we ever need to upgrade versions.

I want to make some changes to the code that places an order, by using a partial class in the same assembly:

Orignal Source Code:

namespace Nop.Services.Orders {

  public partial class OrderProcessingService : IOrderProcessingService {

        public virtual PlaceOrderResult PlaceOrder(ProcessPaymentRequest processPaymentRequest)
        { //....

My partial class:

namespace Nop.Services.Orders {

  public partial class OrderProcessingService : IOrderProcessingService {

    public override PlaceOrderResult PlaceOrder(ProcessPaymentRequest processPaymentRequest) { //....

When I try to compile this code I get an error:

Type 'Nop.Services.Orders.OrderProcessingService' already defines a member called 'PlaceOrder' with the same parameter types

But I am using override and the method in the original class is virtual, could someone tell me where I am going wrong here, and how I could override this method?