Why were product swaps not applied on prepaid subscriptions?

The product swap can only swap products before the subscription renewal. And in prepaid subscriptions, the customer already paid for the specific items in advance, so the system can't swap it anymore. A workaround for this is to set up a non-prepaid subscription with a minimum number of payments. This way, customers won't be able to cancel the subscription until they reach your requirements and the product swaps will still be able to swap products before each subscription renewal.