<p>Is it possible to change a sql server instance name?  Or is it something that can only be set during installation?</p>

To the best of my knowledge, they can only be changed at install time.  You might be able to change the name with the installer package while keeping the current info.  I would make backups of all of your databases and then try this.
On another note, changing instance names will just cause you so many headaches.  Even if you can do it, I would strongly reccomend leaving well enough alone.