Once you upgrade your tools to SQL Server 2005, you can't manage by default SQL Server 2000 packages. The answer is to download a piece of the SQL Server 2005 Feature Package (http://download.microsoft.com/download/4/4/D/44DBDE61-B385-4FC2-A67D-48053B8F9FAD/SQLServer2005_DTS.msi), which includes the DTS Designer. After this installation, you can update packages just like you were in Enterprise Manager. They didn't include this by default to reduce the surface area of the installation.
After you upgrade your SQL Server instance you'll be able to find your 2000 packages in the Microsoft SQL Server Management Studio under the Management node then Legacy -> Data Transformation Services. In an upgrade of an instance, packages and jobs don't move so you can slowly upgrade your DTS packages using the Upgrade Wizard.
-- Brian Knight