That kinda depends on how you set up your project. Are you using events, listeners, and dependency injection extensively? If so, you might need to look at a Symphony/Doctrine2 combo or perhaps Zend Framework 2/Doctrine2. I came to Phalcon from ZF2, and the transition is not extremely drastic. Having said that, if you have a complicated project that uses things like Phalcon's Form implementation, you will have quite a bit of work ahead of you regardless. If the app is a very vanilla MVC app, you might be in better luck, but it will still be painful.
Good luck with a conversion. It would have been nice is the customer had indicated ahead of time that they wouldn't change their PHP configuration!