Old migration doesn’t require downtime

db/migrate/20160610201627_migrate_users_notification_level.rb

@@ -2,6 +2,8 @@ class MigrateUsersNotificationLevel < ActiveRecord::Migration
   # Migrates only users who changed their default notification level :participating
   # creating a new record on notification settings table
+  DOWNTIME = false
+
   def up
     execute(%{
       INSERT INTO notification_settings