Happy Scale, a weight tracking app I really enjoy, had the opposite example: I bought a full license for some bucks some years ago, and later they decided to switch to a subscription based model to ensure funding. Instead of going the "hey, here is a new version of the app, you need to subscribe now", they explicitly included a thank-you note directed at everyone that bought the "Premium In-App Purchase" at the begin…
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.