SLES for SAP Marketplace Subscription transferable to other instance families or instance sizes?

The SLES for SAP subscription from the AWS Marketplace is not transferable between instance types. The purchase is for a particular instance type and family. Similar to an RI, it will apply the discount to any instance of that particular instance type in the account. If you change the instance type or family, the discount will not apply to the changed version.

As long as the AMI is the one owned by AWS (not the old one managed by Orbitera/SUSE), then you can request a refund for the annual subscription, which is pro-rated based on the remaining term of the subscription, as long as a new annual subscription is purchased for the new instance type. The term date starts from the day of purchase. The link provided in the question is the version sold by AWS, so this applies to them.

On a separate note, it should be mentioned that upgrading the OS type from version to version, such as SLES for SAP 12 SP2 to SP3, or even SLES 12 SP3 to SLES 15 SP1, will not have an impact on the reservation. The product code between versions is the same, so customers should feel free to upgrade from within the instance using zypper.