Can I rename an instance of SQL Server after I have already installed it on the computer? Simple answer is Yes and you can change the name of an instance as long as the server is not clustered. There is a little catch between 2000 and 2005 version, by default when you change the name of the computer...
If you are managing hundreds of SQL Server and you might have organised your Management Studio screen with relevant server groups under Server registration. In this case if a new starter has joined and required to assist you in monitoring all of the SQL Server you are taking care, you wouldn't attempt...