public function uppercaseAction()
{
    // get customer collection
    $customers = Mage::getModel('customer/customer')->getCollection()->addAttributeToSelect(array('firstname'), 'inner');
    // call iterator walk method with collection query string and callback method as parameters
    Mage::getSingleton('core/resource_iterator')->walk($customers->getSelect(), array(array($this, 'customerCallback')));
}

// callback method
public function customerCallback($args)
{
    $customer = Mage::getModel('customer/customer’); // get customer model
    $customer->setData($args['row']); // map data to customer model
    $customer->setFirstname(strtoupper($customer->getFirstname())); // set value of firstname attribute
    $customer->getResource()->saveAttribute($customer, 'firstname'); // save only changed attribute instead of whole object
}

Mage::getSingleton('core/resource_iterator')->walk($customers->getSelect(), array($this, 'customerCallback'));