睇返英文版:
If you are changing your existing app to a subscription-based business model, you should not take away the primary functionality existing users have already paid for. For example, let customers who have already purchased a “full game unlock” continue to access the full game after you introduce a subscription model for new customers.
呢條guideline係用should not而唔係must not, 可能就係因為咁Apple先俾佢過