You can replicate the code in my original example in an IEX interactive shell.

I’ve done some more playing about with it in iex and here is the process I took:

defmodule Order do
  use Ecto.Schema

  schema "orders" do
    embeds_many :items, Item, on_replace: :delete #added on_replace to allow manipulation
  end
end

defmodule Item do
  use Ecto.Schema

  embedded_schema do
    field :title
  end
end

With zero items attached to the order:

order = %Order{}
changesetWithNoItems = Ecto.Changeset.change(order)

changesetWithNoItems.data
%Order{__meta__: #Ecto.Schema.Metadata<:built, "orders">, id: nil, items: []}

# this doesn't make sens
changesetWithNoItems |> Ecto.Changeset.put_change(:items, [])
#Ecto.Changeset<
  action: nil,
  changes: %{items: []},
  errors: [],
  data: #Order<>,
  valid?: true
>

# Makes sense
changesetWithNoItems |> Ecto.Changeset.put_change(:items, nil)
#Ecto.Changeset<
  action: nil,
  changes: %{},
  errors: [items: {"is invalid", [type: {:array, :map}]}],
  data: #Order<>,
  valid?: false
>

# Makes sense 
changesetWithNoItems |> Ecto.Changeset.put_change(:items, [%Item{id: 1, title: "test"}])
#Ecto.Changeset<
  action: nil,
  changes: %{
    items: [
      #Ecto.Changeset<action: :insert, changes: %{}, errors: [], data: #Item<>,
       valid?: true>
    ]
  },
  errors: [],
  data: #Order<>,
  valid?: true
>

Then with an order that has a single item attached:

order = %Order{items: [%Item{id: 1, title: "what"}]}
changesetWithItem = Ecto.Changeset.change(order)

changesetWithItem.data 
%Order{
  __meta__: #Ecto.Schema.Metadata<:built, "orders">,
  id: nil,
  items: [%Item{id: 1, title: "what"}]
}

# Makes sense
changesetWithItem |> Ecto.Changeset.put_change(:items, [%Item{id: 1, title: "what"}])
#Ecto.Changeset<action: nil, changes: %{}, errors: [], data: #Order<>,
 valid?: true>

# Doesn't make sense
 changesetWithItem |> Ecto.Changeset.put_change(:items, [%Item{id: 1, title: "huh"}]) 
#Ecto.Changeset<action: nil, changes: %{}, errors: [], data: #Order<>,
 valid?: true>

# confirming
 (changesetWithItem |> Ecto.Changeset.put_change(:items, [%Item{id: 1, title: "huh"}])).data
%Order{
  __meta__: #Ecto.Schema.Metadata<:built, "orders">,
  id: nil,
  items: [%Item{id: 1, title: "what"}]
}

# Doesn't make sense
changesetWithItem |> Ecto.Changeset.put_change(:items, [%Item{id: 2, title: "huh"}])
#Ecto.Changeset<
  action: nil,
  changes: %{
    items: [
      #Ecto.Changeset<action: :replace, changes: %{}, errors: [], data: #Item<>,
       valid?: true>,
      #Ecto.Changeset<action: :insert, changes: %{}, errors: [], data: #Item<>,
       valid?: true>
    ]
  },
  errors: [],
  data: #Order<>,
  valid?: true
>

# Confirming
((changesetWithItem |> Ecto.Changeset.put_change(:items, [%Item{id: 1, title: "huh"}, %Item{id: 2, title: "something new"}])).changes.items |> hd).data
%Item{id: 1, title: "huh"}