From the download page for upgrade advisor "Upgrade Advisor analyzes instances of SQL Server 2000 and SQL Server 2005 to help you prepare for upgrades to SQL Server 2008." This means you run upgrade advisor BEFORE upgrading to see what issue might arise. You have already upgraded your database. You may be running it in compatibility mode 80 but the database has still been upgraded already. If you still have a copy on your SQL 2000 instance run the upgrade advisor there.