Two versions of iOS app - Free and Paid - how to conditionally change project ID in Xcode?

I have my app with ID com.mydomain.AppName which is a paid version.

I decided to introduce free version as well, and through my code I easily add ads/remove some functionality with simple #defined/#ifdef business.

However, I do need my app ID to be different for free version. How do I do this conditionally (i.e. #ifdef FREE_VERSION ... etc.) for my app?