public void DowngradeToCart()

class MyPage : System.Web.UI.Page
{
   public void DoSave(Dynamicweb.eCommerce.Orders.Order order)
   {
       //Broadcast CheckoutDoneOrderIsNotComplete notification
       NotificationManager.Notify(Dynamicweb.eCommerce.Cart.Notifications.CheckoutDoneOrderIsNotComplete, new Dynamicweb.eCommerce.Cart.Notifications.CheckoutDoneOrderIsNotCompleteArgs { Order = order });

       //Convert back to cart and add to context
       order.DowngradeToCart();
       Dynamicweb.eCommerce.Common.Context.SetCart(order);
       order.CartV2StepIndex = 0;
       order.Save(order.ID);
   }
}