How to limit entity reference for new nodes only?
You need to render views in the Entity Reference field. 


Add a reference type display in a view.
Add a nid and title field in the view.
A list of views will be displayed in the field configuration. Select specific view in field's configuration.




Edit: 

May be adding hook_form_alter to form can do the trick for you.

function hook_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'your_node_form') {
    // $form['#node'] if it is node edit form.
    // $form['reference_field']['#options'] = $altered_list
  }
}