In a mixed-version environment, SAP Replication Server must be version 12.6 and later.
If a replication system domain has any SAP Replication Server version 15.5 and later, the system version and route versions in the replication system domain must be version 12.6 and later. SAP Replication Server 15.5 or later does not support mixed-version environments containing versions earlier than 12.6.
See Requirements for Upgrading in the Configuration Guide.