Assuming the plan is to keep/upgrade two of the five replica servers and decommission the other three...
When to decommission the replicas depends on the business requirements. Just make sure all files are synchronized/replicated back to the master and there are no files in the replica cache vaults before decommissioning.
- If the extra replicas aren't necessary today, decommission them before upgrade. This will reduce the number of file pointers in your DB and speed the upgrade pass a little.
- If the replicas are necessary, decommission them after upgrade, before go-live.
Replicas can be installed anytime, but remember that upgrades can take a long time (several months) from initial setup to final production pass, and incrementally patching replica servers is time consuming. Replica interaction/performance validation also needs to occur before go-live.
- If there are no plans to keep up with CPS patches during the upgrade period, install the two new replica servers as part of the production environment up front.
- If CPS patching is likely, I'd wait to install the replica servers until ready to schedule the production pass. That way time isn't spent patching production and two replicas mid-way through the upgrade process and replica validation can still occur before the final production pass.
Once the upgraded production DB is in place, fix the remaining replica configurations, including the security keys, and broadcast the configuration. Copy the remaining replica contents from old replica servers to the new ones, validate the replica vaults, and remove unreferenced files. Then it should be ready for validation testing.
