That worked for me really well:
drush scr update_field_description_100.php
update_field_description.php
$field_name = "field_description_100";
$rows = db_query("SELECT id,data FROM {field_config} WHERE field_name = '{$field_name}' ")->fetchAll(PDO::FETCH_OBJ);
foreach ($rows as $row ){
$row->data=unserialize($row->data);
unset($row->data['settings']['max_length']);
$row->type="text_long";
drupal_write_record("field_config", $row, "id");
}
// Update field_config_instance
$rows = db_query("SELECT id,data FROM {field_config_instance} WHERE field_name = '{$field_name}'")->fetchAll(PDO::FETCH_OBJ);
foreach ($rows as $row ){
$row->data=unserialize($row->data);
$row->data['widget']['type']='text_textarea';
unset($row->data['widget']['settings']['size']);
$row->data['widget']['settings']['rows']=3;
drupal_write_record("field_config_instance", $row, "id");
}
// Update the field table
db_change_field("field_data_{$field_name}", "{$field_name}_value", "{$field_name}_value", array(
"type" => "varchar",
"length" => "20000",
));
db_change_field("field_revision_{$field_name}", "{$field_name}_value", "{$field_name}_value", array(
"type" => "varchar",
"length" => "20000",
));
// Clear caches.
field_cache_clear(TRUE);
Manage fields:

Content editor:

Taken from:
http://thedombroshow.com/drupal-7-converting-a-text-field-to-a-long-text-aka-textarea-field/