Update selected shipping rate if there is an original shipping method to keep and it is different from the one selected through the Estimator process

Make sure the shipment is saved (callbacks!) whenever the ship method has changed in the refresh_rates process

app/models/spree/shipment.rb

@@ -108,19 +108,35 @@ module Spree
 
       # The call to Stock::Estimator below will replace the current shipping_method
       original_shipping_method_id = shipping_method.try(:id)
-
       self.shipping_rates = OrderManagement::Stock::Estimator.new(order).shipping_rates(to_package)
 
-      if original_shipping_method_id
-        selected_rate = shipping_rates.detect { |rate|
-          rate.shipping_method_id == original_shipping_method_id
-        }
-        self.selected_shipping_rate_id = selected_rate.id if selected_rate
-      end
+      keep_original_shipping_method_selection(original_shipping_method_id)
 
       shipping_rates
     end
 
+    def keep_original_shipping_method_selection(original_shipping_method_id)
+      return if shipping_method&.id == original_shipping_method_id
+
+      rate_for_original_shipping_method = find_shipping_rate_for(original_shipping_method_id)
+      if rate_for_original_shipping_method.present?
+        self.selected_shipping_rate_id = rate_for_original_shipping_method.id
+      else
+        # If there's no original ship method to keep, or if it cannot be found on the ship rates
+        #   But there's a new ship method selected (first if clause in this method)
+        #   We need to save the shipment so that callbacks are triggered
+        save!
+      end
+    end
+
+    def find_shipping_rate_for(shipping_method_id)
+      return unless shipping_method_id
+
+      shipping_rates.detect { |rate|
+        rate.shipping_method_id == shipping_method_id
+      }
+    end
+
     def currency
       order ? order.currency : Spree::Config[:currency]
     end

spec/models/spree/shipment_spec.rb

@@ -126,7 +126,8 @@ describe Spree::Shipment do
 
       it 'should request new rates, and maintain shipping_method selection' do
         OrderManagement::Stock::Estimator.should_receive(:new).with(shipment.order).and_return(mock_estimator)
-        shipment.stub(shipping_method: shipping_method2)
+        # The first call is for the original shippping method, the second call is after the Estimator was executed
+        allow(shipment).to receive(:shipping_method).and_return(shipping_method2, shipping_method1)
 
         expect(shipment.refresh_rates).to eq shipping_rates
         expect(shipment.reload.selected_shipping_rate.shipping_method_id).to eq shipping_method2.id