You could try the script here. It worked for me to revert a 2.0 install to 1.5.2. It was a very minimal set of pages, but seems to work for others except one – see comments.
I think you have your backup of the database – be sure before you try anything like this.
http://somethingunpredictable.com/archives/29/12/2005/wordpress-database-downgrader/
