I'm not sure if this is something we need to worry about in this case but this migration won't rollback. As in `rake db:rollback` does nothing after applying this migration. If we want to support rollback with this migration we should probably use `up` and `down` or if we want to use `change` if you use `add_column :cookbook_versions, :legacy_id, :integer` I believe it magically figures out how to rollback. Not sure if add_index does the same or not...