It is not possible. BIDS 2008 projects are not downgradeable to 2005.
If you're an expert in XML, and havea lot of time on your hands, you may be able to directly view 2005 and 2008 package XML, determine the differences that make a 2008 package different from a 2005 package, then edit the 2008 package's XML directly to downgrade it. I wish you luck.
