Hi @amstatonline,
It's not a good practice to upgrade Zend Framework instead of Magento. Magento as platform is very dependent on the set of Zend Framework components and each Magento version addresses specific vulnerabilities from each Zend Framework version (there are even specific Magento patches for ZF).
I'd suggest you to keep Magento updated on its latest version, as well as keeping all the 3rd-party extensions updated.
Best regards.
Gabriel