Upgrade from SBS 2003 to Standard 2003

Hi

I have a small business running SBS 2003 (Prem). We only use our server as a print and file. As the CALS for SBS are so pricey we want to change the role of main server ie domain controller and active directory to your new server which is faster and more powerful. The new server is running Standard 2003 R2.

So heres my question, what is a quick and easy way to transfer users to the new server with out too much down time and a lot of hastle?

Thanks a million for any help!!!