A replication system can include Replication Servers or Adaptive Servers of different versions. Each system presents different issues.
If a replication system domain has Replication Server 15.5 and later, then the system version, and all site and route versions in the replication system domain, must be version 12.6 and later.
You must upgrade Replication Server to version 12.6 or later, set site version to 12.6 or later, and upgrade routes to 12.6 or later, before you can upgrade to version 15.5 or later.
See Replication Server Configuration Guide > Upgrade or Downgrade Replication Server.
When all Replication Servers are at least version 12.6 and the system version is set to 12.6, each Replication Server uses features according to its site version. For example, Replication Servers running version 15.5 can use all 15.5 features among themselves, while Replication Servers running 15.0 can only use 15.0 features. Such a system is called a mixed-version system; each Replication Server can use all of its features.