Overcast (Marco Arment's podcast app for iOS) did something similar. Originally it was an ad-free freemium app. A one-time IAP unlocked premium features (£2.99

Overcast (Marco Arment's podcast app for iOS) did something similar. Originally it was an ad-free freemium app. A one-time IAP unlocked premium features (£2.99). Then all features we made free for all users. (People who already paid were no worse off.) Then ads were added, even for people who had paid the one-off £2.99 early on. The only way to get rid of them is to pay an annual subscription fee. The amounts are small, and the app is great. It's not easy to keep everyone happy and build a sustainable business at the same time.