struct_change.ex

defmodule Tester do
  defstruct [:first, :second]

  def foo_to_bar(struct = %{__struct__: struct_module}) do
    updated_list =
      struct
      |> Map.from_struct
      |> Enum.map(&transform_value/1)
      struct(struct_module, updated_list)
  end

  defp transform_value({key, :foo}), do: {key, :bar}
  defp transform_value({key, value}), do: {key, value}
end

iex(2)> s = %Tester{first: :foo, second: :baz}
%Tester{first: :foo, second: :baz}
iex(3)> Tester.foo_to_bar(s)
%Tester{first: :bar, second: :baz}