Testing configuration Changes

What is the best way to test configuration changes in a “non-production environment” (meaning we can’t just use the system for a couple of days to see what happens).

Configurations could include removing/adding software, diabling services, firewall changes, port redirection, changing security settings, implementing local user policies/groups, installing virus protection, etc.

Once all this is done to a devices that has had no security for years, how can I test it and make sure no loss of functionality has occured? Again, the test has to be in a non-production environment.

Recommendations?