Allow customers to switch subscription plans from the portal

Allow subscribers to change/switch their subscription plan to a different one directly from the customer portal (both hosted and embedded). Current behavior: Customers can swap products, change quantity, and modify frequency from the portal, but they cannot switch to a different subscription plan entirely. Use case: Merchant sells subscription boxes (not individual products) and wants customers to be able to upgrade/downgrade or switch between different subscription plans without needing to cancel and resubscribe.