One-click subscription upgrade/downgrade (without cancellation + new)

As a coach, I want a client to be able to upgrade or downgrade an existing subscription themselves within the mobile app, without cancelling + starting a new one. The options the client sees are fully determined by rules set per subscription (allowed transitions). Before confirming, the app clearly displays: the new price, the effective date (immediate or next term), any pro-rata for an upgrade, and what happens to the price difference/credit upon downgrade (e.g., credit, no refund). Optionally, the coach can set whether a change is implemented immediately or requires approval first. After the change, both the coach and the client receive a confirmation, and the change is logged with the subscription.