Is it safe to change the IP address of the machine running the QESSO Controller?
The QESSO behavior is not influenced by the IP address. Only the server DNS and NetBIOS names have an impact on it.
The IP address change can be done safely, as long as the DNS entries for the server are updated aswell. Be aware that client may be caching the old DNS entry so it's advised to run ipconfig /flushdns on the clients - or what for the cache entry to expire.