Conditionally change field widget type with Drupal hook_form_alter

I want to conditionally change the widget of a plain text field attached to a node by using hook_form_alter in Drupal 8.7.6.

function mymodule_form_node_article_edit_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  $options=[
    'test1'  => 'AAA',
    'test2'  => 'BBB',
    'test3'  => 'CCC',
    'test4'  => 'DDD',
  ];
  $form['field_testdrop']['widget'][0]['#type']='select';
  $form['field_testdrop']['widget'][0]['#options'] = $options;

}

However, when trying to save the node, I get the following error:

Error: Cannot create references to/from string offsets in Drupal\Component\Utility\NestedArray::setValue() (line 155 of core/lib/Drupal/Component/Utility/NestedArray.php).

What can I do to prevent this?

This Works:

    $form['field_priority_email_id']['widget'][0]['value']['#type'] = 'select';
    $form['field_priority_email_id']['widget'][0]['value']['#options'] = $options;
    $form['field_priority_email_id']['widget'][0]['value']['#size'] = NULL;

You have to set '#size' to NULL or the select widget will not render correctly.