It is not possible to do a downgrade as far as I know. An ***upgrade*** is not a problem. You should know that the DVD edition is not important. You can use the Enterprise Edition DVD just enter the Standard Edition key at install time, and only Standard Edition will be installed.
I haven't done this with SQL 2008 however I have plenty of times with 2000 and 2005, the only option then was to uninstall the instance and reinstall. There are lots of articles out there detailing step by step what to do. Just make sure you have all your jobs, users, SSIS, etc scripted out.