Upgrade Windows 2003 TS CAL to Windows 2012 RDS CAL?

Hi there,

I like to understand if a) the upgrade can be done through MS Licensing Clearing house and b) if 'it is cheaper' or if there are any other benefits to do so or we should just consider getting Windows 2012 RDS CAL licenses altogether?

The scenario is that the client is moving towards a Windows 2012 RDS with VDI solution and they have like some 500-2000 Windows 2003 TS CALs between different sites. I believe they are SA customers for licensing purposes.

Thanks for your help in advance!