Try to identify the product ids that have this problem.
If all of them are in this situation that it's easier.
The idea is to first get an all the products that need modifying
If you can get them manually do this:
$ids = array(1,3,5,6,7,8, ...);
$products = Mage::getModel('catalog/product')->getCollection()->addAttributeToSelect('description')->addAttributeToFilter('entity_id', $ids);
if all the products need modifying do this:
$products = Mage::getModel('catalog/product')->getCollection()->addAttributeToSelect('description');
After you have all the ids run this script:
foreach ($products as $product) {
Mage::getSingleton('catalog/product_action')->updateAttributes(
array($product->getId()), //id to update
array( //values to update
'sku' => $product->getDescription(),
'description' => $product->getSku()
),
0 //store id
);
}