SQL Setup Magento - Upgrade table

Come fare l'alter table di colonna?
Io uso questo metodo.

$installer = $this;
$installer->startSetup();
/**
tabella mdg_giftregistry_item
 */
$tableName = $installer->getTable('mdg_giftregistry/item');
/**
 * @var $table Varien_Db_Ddl_Table
 */
$table = $installer->getConnection()->newTable($tableName);
$installer->getConnection()->addColumn($tableName, 'customer_id',
    array(
        'type'=>Varien_Db_Ddl_Table::TYPE_INTEGER,
        'unsigned' => true,
        'nullable' => false,
        'primary' => true,
        'comment'=>'customer_id',
        'primary'=>false
    ));
$installer->endSetup();