Change SQL Server settings without restarting services
It depends on what settings you changed based on http://msdn.microsoft.com/en-GB/library/ms189631.aspx. Some require a restart (RR), other can be done via RECONFIGURE

There is no trick or workaround. At all.

Edit, now we know it is Authentication mode...

This is automatically deals with the instance name

 EXEC xp_instance_regwrite 
    N'HKEY_LOCAL_MACHINE', 
    N'Software\Microsoft\MSSQLServer\MSSQLServer', 
    N'LoginMode', 
    REG_DWORD, 
    1; -- or 2 for mixed


Restart is still needed