How to: Upgrade Database Diagrams from Previous Editions (Visual Database Tools)
Database diagrams created on previous editions of SQL Server databases are upgraded automatically when database diagramming is set up on SQL Server 2008. For more steps for setting up diagramming see How to: Set Up Database Diagram Designer (Visual Database Tools).
To upgrade legacy database diagrams
From Object Explorer, expand the database.
Expand the Database Diagram node under the database.
Select Yes when prompted if you want to set up database diagramming.
This will upgrade diagrams stored on the database.