XML lenses have stumped me.

xml-lens.hs

-- stack --resolver nightly-2017-12-01 script
{-# LANGUAGE OverloadedStrings #-}

import qualified Data.Text.Lazy.IO as LazyText
import Text.XML
import Text.XML.Lens

main :: IO ()
main = do
  let original = parseText_ def "<root> <old>1</old> <old>2</old> </root>"
  let document = original & root ./ named "old" .~ Element "new" mempty []
  LazyText.putStrLn $ renderText def { rsXMLDeclaration = False } document
  -- This prints:
  --   <root> <new/> <new/> </root>
  -- But I want:
  --   <root> <new/> </root>
  -- This is, *all* <old> elements replaced by a *single* <new> element.