Upgrade 2003r2 to 2003 enterprise

I currently have a DC that is running server 2003 r2. I need to upgrade it to the the enterprise version and turn it into a mail server as well thus the upgrade. I allready have the new software so let's skip the server 2008 suggestions. I am wodering if I could do what I will call an in place upgrade. Meaning can I remove it from the networkk and load the new server 2003 enterprise server and just do an upgrade or should I do a full clean install?

Thank you as alwyas...Chris P.

If you don't have another DC to which you can transfer the roles don't think about "upgrading" it. At least not without having a FULL WORKING BACKUP. Things can and usually *DO* go bad... Your best bet is to backup your DC, install a secondary DC, transfer the roles and then upgrade.
Ideally ummm.. go to 2008... 2003 is out of mainstream support - which means no more patches will be released, except critical ones and all support from MS is PAID ONLY.