I put this question to Chat GPT and had a very helpful conversation ... as of Sept 2021 ;-) It would seem my desire for a seamless cost reduction in an on-going subscription is not possible within Apple. Either there is an ongoing higher priced subscription and then after a certain time the user becomes eligible for a new lower cost one (but the user might let the old one run unnecessarily long by mistake, despite in-app warnings and messages, then discover their error and feel aggrieved), or the initial subscription is time-limited to avoid this scenario, but then the user has the inconvenience of being exited from the app if it runs right out, or of cancelling one subscription, and having to re-subscribe to the cheaper rate. If this is indeed correct, I think it is a real shame that this isn't made a simple and convenient option for the user and app-designer.