What is the structure of the SCWCD certification? Is it only an exam based certification or is there an assignment too as with the SCJD certification?
Like SCJP, it is a single multiple-choice/drag-and-drop examination only. There is no practical component.
Secondly what is the difference between the normal SCWCD exam and the upgrade exam?
The upgrade exam is only for people who have passed a previous version of the SCWCD exam (e.g. 1.3 or 1.4). If you have never sat an SCWCD exam before, you need to head for the non-upgrade (310-083) version. The upgrade is slightly shorter (fewer questions) given the candidate is already SCWCD---there are no other differences.
Charles Lyons (SCJP 1.4, April 2003; SCJP 5, Dec 2006; SCWCD 1.4b, April 2004)
Author of OCEJWCD Study Companion for Oracle Exam 1Z0-899 (ISBN 0955160340 / AmazonAmazon UK )