Update backend model of product attribute with setup script

I need to add the backend_model value of an attribute where wasn't specified in the install setup script. The missing line is:

'backend' => 'eav/entity_attribute_backend_datetime'

But so far using just this line in an upgrade script

$installer  = Mage::getResourceModel('catalog/setup', 'catalog_setup');
$installer->startSetup();

$installer->updateAttribute('catalog_product', 'custom_date', array(
'backend' => 'eav/entity_attribute_backend_datetime'));

$installer->endSetup();

Is not updating the column backend_model in the ear_attribute table. So I'm forced to use

$installer  = Mage::getResourceModel('catalog/setup', 'catalog_setup');

$installer->startSetup();

$installer->removeAttribute('catalog_product','custom_date');

$installer->addAttribute('catalog_product', 'custom_date', array(
    'label' => 'Launches - Release Date',
    'type'      => 'datetime',
    'input'     => 'date',
    'class'     => 'validate-date',
    'global'    => Mage_Catalog_Model_Resource_Eav_Attribute::SCOPE_GLOBAL,
    'required'  => false,
    'user_defined' => false,
    'searchable' => false,
    'filterable' => false,
    'comparable' => false,
    'visible_on_front' => false,
    'unique' => false,
    'used_in_product_listing' => true,
    'backend' => 'eav/entity_attribute_backend_datetime'
));

$installer->endSetup();

Any idea if is actually possible to update the backend_model in an upgrade script.