Cool! Sorry for the late response, but I’m glad you figure it out. Like I said, I also remember running into some walls getting it to work the way I though it was supposed to. Like you, I eventually got it working and it has been great for me ever since. Best regards!

Ok, I think I got it to work:

I added a default to the client_type:

    field :client_type, Ecto.Enum, values: [:legal_entity, :individual], default: :legal_entity

Then the new puts the correct embedded struct, depending on the `client_type`:

  defp apply_action(socket, :new, _params) do
    client = %Client{user_id: socket.assigns.current_scope.user.id}

    client =
      case client.client_type do
        :legal_entity ->
          %Client{client | client_name: %LegalEntityName{}}

        :individual ->
          %Client{client | client_name: %IndividualName{}}
      end

    socket
    |> assign(:page_title, gettext("New client"))
    |> assign(:client, client)
    |> assign(:form, to_form(Clients.change_client(socket.assigns.current_scope, client)))
  end

And when validating, if the `client_type` changed, I change the polymorphic_embed struct too:

  @impl true
  def handle_event("validate", %{"client" => client_params}, socket) do
    changeset =
      Clients.change_client(socket.assigns.current_scope, socket.assigns.client, client_params)

    changeset =
      case Ecto.Changeset.get_change(changeset, :client_type) do
        nil -> changeset
        :legal_entity -> Ecto.Changeset.put_change(changeset, :client_name, %LegalEntityName{})
        :individual -> Ecto.Changeset.put_change(changeset, :client_name, %IndividualName{})
      end

    {:noreply, assign(socket, form: to_form(changeset, action: :validate))}
  end

The problem with replacing the client_name on the changeset when `client_type` changes is that the previous values are lost, but is something I can live with.

Thanks!