Upgrade Replication Server to 16.0 SP03 PL05 verification

We are currently running ASE 15.7 SP139 with Replication Server Replication Server/15.7.1/EBF 25284 SP210. We are preparing to upgrade Replication Server to 16.0 SP03 PL05. We have Primary Servers in -- we replicate using MSA replication to Servers in UK . We wanted to verify we can do this in stages -- First upgrading the servers in the UK and then US servers. Just wanted to verify that this can be done incrementally -- where we upgrade Target Rep servers first -- then source. The ASE servers -- which also hold the RSSD databases, will be upgraded at later date to ASE 16. Can we do the upgrades incrementally or do we need to upgrade all at same time Thanks