Add column type to labels and do the batch update in the same migration

db/migrate/20160919144305_add_type_to_labels.rb

 class AddTypeToLabels < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
 
-  DOWNTIME = false
+  DOWNTIME = true
+  DOWNTIME_REASON = 'Labels will not work as expected until this migration is complete.'
 
   def change
     add_column :labels, :type, :string
+
+    update_column_in_batches(:labels, :type, 'ProjectLabel') do |table, query|
+      query.where(table[:project_id].not_eq(nil))
+    end
   end
 end

db/migrate/20160920191518_set_project_label_type_on_labels.rb

-class SetProjectLabelTypeOnLabels < ActiveRecord::Migration
-  include Gitlab::Database::MigrationHelpers
-
-  DOWNTIME = false
-
-  def up
-    update_column_in_batches(:labels, :type, 'ProjectLabel') do |table, query|
-      query.where(table[:project_id].not_eq(nil))
-    end
-  end
-
-  def down
-    update_column_in_batches(:labels, :type, nil) do |table, query|
-      query.where(table[:project_id].not_eq(nil))
-    end
-  end
-end