Wayne Sheffield shows what to do when you need to rename your SQL Server instance:
Sometimes you make a mistake, and forget to rename a syspred’d server before installing SQL Server. Or perhaps your corporate naming standard has changed, and you need to rename a server. Maybe you like to waste the time involved in troubleshooting connection issues after a server rename. In any case, you now find yourself where the name of the SQL Server is different than the physical name of the server itself, and you need to rename SQL Server to match the server’s physical name.
You could always rerun the setup program to rename the server. Fortunately, SQL Server provides an easier way to do this. You just need to run two stored procedures:sp_dropserver and sp_addserver.
Click through for details, including important considerations.