Possible unavailability reasons
The customer’s subscription was purchased with a payment gateway that is not supported by switch functionality.
The customer's subscription is currently during the recurring payment process. Once a new billing cycle begins it will be possible to use switch functionality.
The customer already has an active subscription to the target offer.
Target offer is not accessible in the customer's country. Possible when geo-restriction was enabled on the target offer.
Switch functionality is not supported for offers with price 0.