diff --git a/app/models/standing_line_item.rb b/app/models/standing_line_item.rb
index 6f4372d4cf..70d0eebf82 100644
--- a/app/models/standing_line_item.rb
+++ b/app/models/standing_line_item.rb
@@ -1,4 +1,8 @@
+require 'open_food_network/standing_line_item_updater'
+
 class StandingLineItem < ActiveRecord::Base
+  include OpenFoodNetwork::StandingLineItemUpdater
+
   belongs_to :standing_order, inverse_of: :standing_line_items
   belongs_to :variant, class_name: 'Spree::Variant'
 
@@ -6,6 +10,8 @@ class StandingLineItem < ActiveRecord::Base
   validates :variant, presence: true
   validates :quantity, { presence: true, numericality: { only_integer: true } }
 
+  before_save :update_line_items! # In OpenFoodNetwork::StandingLineItemUpdater
+
   def available_from?(shop, schedule)
     Spree::Variant.joins(exchanges: { order_cycle: :schedules})
     .where(id: variant_id, schedules: { id: schedule}, exchanges: { incoming: false, receiver_id: shop })
diff --git a/app/models/standing_order.rb b/app/models/standing_order.rb
index e27e8be1fd..29db805d57 100644
--- a/app/models/standing_order.rb
+++ b/app/models/standing_order.rb
@@ -3,6 +3,8 @@ require 'open_food_network/standing_order_updater'
 class StandingOrder < ActiveRecord::Base
   include OpenFoodNetwork::StandingOrderUpdater
 
+  before_save :update_orders!
+
   belongs_to :shop, class_name: 'Enterprise'
   belongs_to :customer
   belongs_to :schedule
@@ -26,8 +28,6 @@ class StandingOrder < ActiveRecord::Base
   validate :standing_line_items_available
   validate :check_associations
 
-  after_save :update_orders!
-
   def ends_at_after_begins_at
     if begins_at.present? && ends_at.present? && ends_at <= begins_at
       errors.add(:ends_at, "must be after begins at")
diff --git a/lib/open_food_network/standing_line_item_updater.rb b/lib/open_food_network/standing_line_item_updater.rb
new file mode 100644
index 0000000000..1834448bde
--- /dev/null
+++ b/lib/open_food_network/standing_line_item_updater.rb
@@ -0,0 +1,35 @@
+module OpenFoodNetwork
+  module StandingLineItemUpdater
+
+    attr_accessor :altered_orders
+
+    def update_line_items!
+      altered_orders ||= []
+      attr_names.each do |attr_name|
+        unaltered_line_items(attr_name).update_all(:"#{attr_name}" => send(attr_name))
+        if altered_line_items(attr_name).any?
+          altered_orders |= altered_line_items(attr_name).map(&:order)
+        end
+      end
+    end
+
+    private
+
+    def attr_names
+      [:quantity]
+    end
+
+    def unaltered_line_items(attr_name)
+      return line_items_from_future_and_undated_orders unless persisted?
+      line_items_from_future_and_undated_orders.where("#{attr_name} = (?)", send("#{attr_name}_was"))
+    end
+
+    def altered_line_items(attr_name)
+      line_items_from_future_and_undated_orders - unaltered_line_items(attr_name)
+    end
+
+    def line_items_from_future_and_undated_orders
+      Spree::LineItem.where(order_id: standing_order.future_and_undated_orders, variant_id: variant_id)
+    end
+  end
+end
diff --git a/lib/open_food_network/standing_order_updater.rb b/lib/open_food_network/standing_order_updater.rb
index ef090581a7..1c34084614 100644
--- a/lib/open_food_network/standing_order_updater.rb
+++ b/lib/open_food_network/standing_order_updater.rb
@@ -23,6 +23,10 @@ module OpenFoodNetwork
       end
     end
 
+    def future_and_undated_orders
+      orders.joins(:order_cycle).merge(OrderCycle.not_closed)
+    end
+
     private
 
     def create_order_for(order_cycle_id)
diff --git a/spec/lib/open_food_network/standing_order_updater_spec.rb b/spec/lib/open_food_network/standing_order_updater_spec.rb
index 3b060c161b..35645fef47 100644
--- a/spec/lib/open_food_network/standing_order_updater_spec.rb
+++ b/spec/lib/open_food_network/standing_order_updater_spec.rb
@@ -111,5 +111,16 @@ module OpenFoodNetwork
         end
       end
     end
+
+    describe "changing the quantity of a line item" do
+      let(:standing_order) { create(:standing_order_with_items) }
+
+      it "updates the quantity on all orders" do
+        sli = standing_order.standing_line_items.first
+        sli.update_attribute(:quantity, 4)
+        line_items = Spree::LineItem.where(order_id: standing_order.orders, variant_id: sli.variant_id)
+        expect(line_items.map(&:quantity)).to eq [4]
+      end
+    end
   end
 end