Data is immutable in Elixir. This is why Ecto.Changeset.add_error/4 returns a new changeset that you need use further on:

updated_changeset = Ecto.Changeset.add_error(changeset, :old_password, "invalid current password")
render conn, "password_new.html",
       changeset: %{updated_changeset | action: :insert}

Elixir does allow the rebinding of variables so you can also write it like this:

changeset = Ecto.Changeset.add_error(changeset, :old_password, "invalid current password")
render conn, "password_new.html",
       changeset: %{changeset | action: :insert}