Transfering Windows Server 2003 CAL to Windows Server 2008

I have 25 pack Windows Server 2003 Standard Devices CALS and I thinking of adding a new server to the network to replace the current windows 2003 server based on Windows 2008 Platform. I will still keep the current Windows 2003 Server as a exchange server. Can these current CALS be transfered to Windows 2008? or Do I have to repurchase these CALS for Windows 2008?