v1_group_serializer.rb

module Export
  module V1
    class GroupSerializer < ActiveModel::Serializers
      attributes :items
      
      def items_serializer
        ItemsSerializer
      end

      def items
        docs = object.items.order_by(:position.asc)
        items_serializer.new(docs, special_options).serializable_hash
      end
    end
  end
end

v1_items_serializer.rb

module Export
  module V1
    class ItemsSerializer < ActiveModel::Serializer
      # ...
    end
  end
end

v2_groups_serializer.rb

module Export
  module V2
    class GroupSerializer < ::Export::V1::GroupSerializer
      def items_serializer
        ItemsSerializer
      end
    end
  end
end