Hi B M
If your requirement is to switch interfaces from existing 7 PI servers to test environment then you need to switch your interfaces from existing to new.
Interested to know if downtime of HO server is temporary. If interfaces are configured with buffering then data is buffered locally on the interfaces and sent to PI server once it is online. This will enable you to cover on time. Another approach (PI Collective - High Availability) in your production environment will allow downtime for any Primary and secondary servers.
As long as all of the tags are configured identically on each server and the security is configured appropriately to allow the clients and PI-to-PI interfaces to connect, then the destination PI server on each of the PI-to-PI interfaces would be the only change needed. The downfall to this approach, besides requiring manual intervention to switch between the two servers is that you won't have all of your data on either server without additional work.
If you have a requirement for high availability of the "HO" Pi server though, it would be best to build a PI Collective. When PI servers are configured as members of a collective, most of the configuration including the tag database and security are continuously synchronized. Then as long as the Buffer Subsystem is configured properly on each of the machines with PI to PI interfaces, the data will be streamed to all servers in the collective and both servers. If One PI server is temporarily unavailable then data will be back filled automatically when the server is available again.