Each of our topics comes in two versions: (1) a free version with an optional in-app upgrade, and (2) a pay-up-front "Pro" version (with a yellow banner on the icon). The free version with the upgrade applied is equivalent to the paid Pro version, but they are not interchangeable. Apple's app bundles can only include paid apps, not free apps with in-app purchases, which is why we offer the Pro option. Also, some of our users expressed an interest in a pay-up-front version so they could give the app as a gift, which can’t be done with an in-app purchase. Most of our users prefer being able to try the free version before upgrading to receive the premium features.
If you ever need to reinstall an app you paid for in the past, or install the app on a different device, make sure you install the same version you purchased initially. If you are reinstalling a free version, after installing, go to the Settings tab and tap Restore In-App Purchase. As mentioned above, if you purchased a bundle in the past, you will need to reinstall the Pro version.