server is still on use. And the new virtual server is on a different ip address. What should be done to test this new virtual server using the fully quailed domain name for this application?

Adding or changing a DNS record would redirect all clients resolving that FQDN to the new IP, breaking access for users still relying on the old server.

Browser DNS caches store previously resolved responses from DNS queries and cannot be manually pre-populated with new custom mappings.

Operating systems check the local hosts file for hostname-to-IP mappings before querying DNS servers, so adding the FQDN with the new virtual server's IP address in the client's hosts file directs only that client to the new server. This approach isolates the test to a single machine and avoids disrupting other users who still depend on the old server remaining reachable via the live DNS record.

A server's services file maps service names to port numbers (like /etc/services on Linux) and has no role in hostname-to-IP address resolution.