diff --git a/sites/all/modules/entity_translation/entity_translation.module b/sites/all/modules/entity_translation/entity_translation.module
index fcf7908..a2851fc 100644
--- a/sites/all/modules/entity_translation/entity_translation.module
+++ b/sites/all/modules/entity_translation/entity_translation.module
@@ -469,6 +469,243 @@ function entity_translation_form_field_ui_field_settings_form_alter(&$form, $for
     '#title' => t('Users may translate this field.'),
     '#default_value' => $field['translatable'],
   );
+  $form['field']['warning'] = array(
+    '#type' => 'container',
+    '#states' => array(
+      'visible' => array(  // action to take.
+        ':input[name="field[translatable]"]' => array('checked' => ! $field['translatable']),
+      ),
+    ),
+  );
+  $form['field']['warning']['warning'] = array(
+    '#markup' => t('By submitting this form you will trigger a batch operation. THIS OPERATION COULD RESULT IN LOSS OF DATA. Please ensure that all effected nodes are backed up before proceeding.'),
+  );
+
+  // our submit handler must run before field_ui_field_settings_form_submit()
+  if (isset($form['#submit'])) {
+    array_unshift($form['#submit'], 'entity_translation_translatable_submit');
+  }
+  else {
+    $form['#submit'] = array('entity_translations_translatable_submit');
+  }
+}
+
+/**
+ * This submit handler maintains consistency between the translatability of a node and the language under which the
+ * field data is stored.
+ *
+ * When a field is marked as translatable, all data under $node->{field_name}['und'] is moved to
+ * $node->{field_name}[$node->language]. When a field is marked as untranslatable, the opposite transfer occurs.
+ *
+ * N.B. When marking a field as untranslatable, all translations of the field (except for $node->language) will be
+ * lost! This should be a very rare case, but a lot of anger will eventually come from it.
+ *
+ * TODO: Make a confirmation page as when anything else is deleted.
+ *
+ * For lack of a better place, notes go here:
+ *
+ * -Make sure that the confirmation screen/message don't appear when creating a new field.
+ */
+function entity_translation_translatable_submit($form, &$form_state) {
+  $checked = $form_state['values']['field']['translatable'];
+  $prior = $form['field']['translatable']['#default_value'];
+
+  if ($prior == $checked) {
+    // Nothing to do.
+    return;
+  }
+
+  $field_name = $form_state['values']['field']['field_name'];
+
+  $ops = array(array('entity_translation_translatable_batch', array($checked, $field_name)));
+
+  // When making field untranslatable, we need to make sure that the other submit handler
+  // can't see the changed translatable setting. We change it ourselves after the batch operation.
+  //
+  // For some reason, adding the custom submit handler to the batch job doesn't work; it never gets called.
+  // Why this is is yet to be determined, but a temporary workaround is to pass the data into the batch results
+  // and then call the submit handler from the finalising callback.
+  if (! $checked) {
+    $ops[] = array('entity_translation_translatable_batch_data_store', array($form, $form_state));
+    $form_state['values']['field']['translatable'] = $prior;
+  }
+
+  // Somehow this submit handler doesn't get called unless it comes first. So, we look through the list and
+  // if we find one, we call it ourselves and remove it from the list. It is removed and executed at most once.
+  //
+  // This is not a good solution, but it works. fixme if you see how.
+  foreach ($form['#submit'] as $key => &$value) {
+    if ($value === 'field_ui_field_settings_form_submit') {
+      unset($value);
+      field_ui_field_settings_form_submit($form, $form_state);
+      break;
+    }
+  }
+
+  $batch = array(
+    'title' => t('Converting'),
+    'operations' => $ops,
+    'finished' => 'entity_translation_translatable_batch_done',
+  );
+
+  batch_set($batch);
+}
+
+/**
+ * Hacky helper function to pass the form data off to entity_translation_translatable_batch_done.
+ */
+function entity_translation_translatable_batch_data_store($form, $form_state, &$context) {
+  $context['results'] = array($form, $form_state);
+}
+
+/**
+ * Function from which to build batch array.
+ */
+function entity_translation_translatable_batch($make_translatable, $field_name, &$context) {
+  if (empty($context['sandbox'])) {
+    $context['sandbox']['progress'] = 0;
+
+    // How many entities will need processing?
+    // Unfortunately EFQ->count() counts all translations of all fields, not just the entities that have them
+    $count = 0;
+    $EFQ = new EntityFieldQuery();
+    $entities = $EFQ->fieldCondition($field_name)
+                 ->execute();
+
+    foreach($entities as $type => $list) {
+      $count += count($list);
+    }
+
+    if ($count === 0) {
+      // No-op.
+      $context['finished'] = 1;
+      return;
+    }
+
+    $context['sandbox']['max'] = $count;
+  }
+
+  // Number to process at a time. Tune as neccessary.
+  $inc = 10;
+
+  $EFQ = new EntityFieldQuery();
+  $query = $EFQ->fieldCondition($field_name)
+               ->range($context['sandbox']['progress'], $inc)
+               ->execute();
+  dsm($query);
+  foreach ($query as $entity_type => $entities) {
+    field_attach_load($entity_type, $entities, FIELD_LOAD_CURRENT, array('field_id' => $field_name));
+  }
+
+  foreach ($query as $entity_type => $entities) {
+
+    // Getting the default language of the entities is still an open problem.
+    // When turning off translatability, the obvious place to look is the source language
+    // attribute in the entity_translation table, but that table doesn't exist until after
+    // the field in question has been made translatable, so when turning translatability
+    // on...
+    //
+    // I think the best thing to do is implement a hook_..._language_alter since the site admins
+    // likely have a much better idea of what language things are than Drupal does.
+    //
+    if (!$make_translatable) {
+      // This exploits the fact that in the entity_translation table, if the source is blank then the
+      // language is the source language.
+      $language = db_select('entity_translation', 'et')
+                ->fields('et', array('entity_id', 'language'))
+                ->condition('source', '', '=')
+                ->condition('entity_id', array_keys($entities), 'IN')
+                ->execute()
+                ->fetchAllKeyed();
+    }
+    else {
+      // Far from ideal interim solution. fixme
+      $language = array_fill_keys(array_keys($entities), language_default()->language);
+    }
+
+    foreach ($entities as $id => $entity) {
+
+      $lang = $language[$id];
+      dsm($lang);
+      if ($make_translatable && isset($entity->{$field_name}[LANGUAGE_NONE])) {
+        // If field is to become translatable and has data in 'und'
+
+        $entity->{$field_name}[$lang] = $entity->{$field_name}[LANGUAGE_NONE];
+        //setting this to an empty array() will cause the item to be deleted
+        $entity->{$field_name}[LANGUAGE_NONE] = array();
+      }
+      else if (!$make_translatable && isset($entity->{$field_name}[$lang])) {
+        // the field has been marked untranslatable and has data in the (entity's) default language
+        // mv the default language to und and drop the other translations.
+        //Store the field data in the default language
+        $data = $entity->{$field_name}[$lang];
+        // Remove all translations of the field data.
+        foreach ($entity->{$field_name} as $langcode => $item) {
+          $entity->{$field_name}[$langcode] = array();
+        }
+        // Move the original field data to LANGUAGE_NONE
+        $entity->{$field_name}[LANGUAGE_NONE] = $data;
+      }
+      else {
+        // Don't save unchanged entities.
+        continue;
+      }
+      // Save field data. Faster than node_save, but still too slow for comfort.
+      field_attach_update($entity_type, $entity);
+    }
+  }
+  $context['sandbox']['progress'] += $inc;
+  if ($content['sandbox']['progress'] < $content['sandbox']['max']) {
+    $context['finished'] = $content['sandbox']['progress'] / $content['sandbox']['max'];
+  }
+}
+
+/**
+ * Check the exit status of the batch operation, and possibly resubmit the changed translatable status.
+ */
+function entity_translation_translatable_batch_done($success, $results, $operations) {
+  if (isset($results) && count($results) === 2) {
+    // Run our custom submit handler.
+    entity_translation_custom_submit($results[0], $results[1]);
+  }
+  if ($success) {
+    drupal_set_message(t("Data successfully converted."));
+  }
+  else {
+    // TODO: Do something about this case.
+    drupal_set_message(t("Something went wrong when converting the data. Some nodes may appear to have lost fields."));
+  }
+}
+
+/**
+ * This is a shameless cut and paste of field_ui_field_settings_form_submit from field_ui.admin.inc.
+ *
+ * The reason is that we need to submit the form, but keep one property (translatabe) aside to be updated
+ * after we've run our batch operation; if we make a field untranslatable before moving the data we lose it.
+ *
+ * TODO: Find the right way to do this.
+ */
+function entity_translation_custom_submit($form, &$form_state) {
+  $form_values = $form_state['values'];
+  $field_values = $form_values['field'];
+
+  // Merge incoming form values into the existing field.
+  $field = field_info_field($field_values['field_name']);
+
+  $entity_type = $form['#entity_type'];
+  $bundle = $form['#bundle'];
+  $instance = field_info_instance($entity_type, $field['field_name'], $bundle);
+
+  // Update the field.
+  $field = array_merge($field, $field_values);
+
+  try {
+    field_update_field($field);
+    drupal_set_message(t('Updated field %label translatable setting.', array('%label' => $instance['label'])));
+  }
+  catch (FieldException $e) {
+    drupal_set_message(t('Attempt to update field %label failed: %message.', array('%label' => $instance['label'], '%message' => $e->getMessage())), 'error');
+  }
 }
 
 /**