Why Apple had a secret meeting with app developers to discuss the App Store
businessinsider.de
businessinsider.de
Given that model, you aren’t risking much by daring to ask your customers to opt in, as literally any other app they can buy would have the same deal. And customers wouldn’t risk much either, as worst-case they pay your initial price and have to keep what they have.
A really nice feature, if Apple is serious about this, would be to have a “resume” price that kicks in based on how long it’s been since the customer last paid. That way, feel free to stop paying but don’t expect to come back in 2 years and get all the latest updates for nothing.