Implements hook_field_update_forbid().
function options_field_update_forbid($field, $prior_field, $has_data) {
if ($field['module'] == 'options' && $has_data) {
// Forbid any update that removes allowed values with actual data.
$lost_keys = array_diff(array_keys($prior_field['settings']['allowed_values']), array_keys($field['settings']['allowed_values']));
if (_options_values_in_use($field, $lost_keys)) {
throw new FieldUpdateForbiddenException(t('A list field (@field_name) with existing data cannot have its keys changed.', array(
'@field_name' => $field['field_name'],
)));
}
}
}