>I think you're going to be stuck with a reinstall. I've never seen anything that indicates that you can convert a named instance of SQL Server to a default instance. Plus this FAQ (http://support.microsoft.com/default.aspx?scid=kb;en-us;260414) indicates that you can't even change the instance name without reinstalling.
>

I checked the BOL and found this.

sqlservr Application
The sqlservr application starts, stops, pauses, and continues an instance of Microsoft® SQL Server™ 2000 from a command prompt.

Syntax
sqlservr [-sinstance_name] [-c] [-dmaster_path] [-f] 
    [-eerror_log_path] [-lmaster_log_path] [-m]
    [-n] [-Ttrace#] [-v] [-x] [-g number] [-O] [-y number]

So with this could you stop the old ones and start a new one?