I don't think you can succeed in changing the default name. It involves changes on the DNS, IIS and other configurations as well - which I am not sure of.
A better option would be to create another site, and point WSS to the new site. No doubt, you will have to rebuild and re-configure for all other stuff. And once you have done this, you need to edit the group policy to ensure that the default website name is changed. You can do this in the "Group Policy Management" control panel applet. It can be found under "Adminstrative Tools" if I am not wrong.
Under the domains, pick the domain and go to Group policy objects. You should find an option that reads "Windows SBS User Policy". On a right-click, you should get options to edit. Somewhere under that you will find the defaultwebsite mentioned. Change the url.
This should help.