Hi cchin,
I can't search of any documentary about this either.. but I think the answer should be OK due to backward compatibility.
Anyway, VS2005 can migrate any project created from VS2003, and similarly VS2008 can migrate any project created from VS2005. VS2008 have multi-version targetting and powerful. Most importantly, VS2003 is buggy so it maybe a good reason for you to upgrade now.. VS2003 is a product created 5 years ago..

Regards,
Colt