We offer all our topics in a free version with an in-app upgrade as well as a paid "Pro" version (with a yellow banner on the icon). After you upgrade the free version, it is equivalent to the paid Pro version, but the two versions are not interchangeable. Due to the way the App Store works, it is not possible for us to make them interchangeable.
The advantage of the upgradeable free apps is that you can try them out to see if they meet your needs before upgrading.
If you think you might be interested in a bundle in the future, we suggest you purchase the Pro version instead of upgrading the free version via an in-app purchase.
If you've purchased an upgrade to a free app and you would like to switch to the Pro version so that you can purchase a discounted bundle, please contact support.