New version theme is not working in old version.
After upgrading magento or when I design theme in magento 126.96.36.199 and use that theme in magento 1.4.2 that time my product page shows error like
Invalid method Mage_Catalog_Model_Product::isDuplicable
I find the solutin and its work for me.
Copy your product.php file from this path: app\code\core\Mage\Catalog\Model\product.php and replace in your maegnto folder.