I can't really find any documentation behind it but as long as the wsus 3.0 sp2 builds match up it should work but just to be safe make an export of the metadata and keep it on a share outside of your server. worst case it will re-download the
actual update files but your approvals will be retain in the export file
a database backup would not hurt either
remember that 2003 has to be x64 with SP2 to upgrade to 2008 R2