struct_change.ex

iex(1)> defmodule Tester do
...(1)> defstruct [:first, :second]
...(1)> def foo_to_bar(struct = %{__struct__: struct_module}) do
...(1)>   updated_list = struct
...(1)>                  |> Map.from_struct
...(1)>                  |> Enum.map(fn({key, value}) ->
...(1)>                    if value == :foo do
...(1)>                      {key, :bar}
...(1)>                    else
...(1)>                      {key, value}
...(1)>                    end
...(1)>                  end)
...(1)>   struct(struct_module, updated_list)
...(1)> end
...(1)> end
{:module, Tester,
 <<70, 79, 82, 49, 0, 0, 11, 100, 66, 69, 65, 77, 69, 120, 68, 99, 0, 0, 0, 243,
   131, 104, 2, 100, 0, 14, 101, 108, 105, 120, 105, 114, 95, 100, 111, 99, 115,
   95, 118, 49, 108, 0, 0, 0, 4, 104, 2, ...>>, {:foo_to_bar, 1}}
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}