There seems to be the open source zend optimizer plus that even works with php 5.4. Will 5.4 be optionally available once you upgrade to PHP5.3?
Zend Optimizer doesn't exist for PHP 5.3.
The product was renamed ZendGuard when it was released for PHP 5.3. However, ZendGuard cannot load files that were encoded for Zend Optimizer, and vice versa -- they're effectively two separate products. If you've got an application that was encoded for Zend Optimizer, you're stuck on PHP 5.2.
You'll need to get with "Sunshop" and have them provide you a version that is compatible with PHP 5.3.