OldFoo
|> Repo.stream()
|> Stream.each(fn foo ->

  new_foo =
     NewFoo
     |> where([nf], nf.name == ^foo.name)
     |> Repo.one()

  Bar
  |> where([b], b.foo_id == ^foo.id)
  |> Repo.stream()
  |> Stream.each(fn bar ->
    Bar.update(bar, %{foo_id: new_foo.id})
  end
  |> Stream.run
end)
|> Stream.run