The primary key or Unique constraint cannot be changed while relationships to the existing primary key or Unique constraint are enforced. (Visual Database Tools)
You cannot change a table's primary key or unique constraint while other foreign key relationships are still in place. To change a table's primary key or unique constraint, first delete all existing relationships for the table.
For more information, see How to: Delete Relationships (Visual Database Tools).
See Also
Concepts
Error Messages in Visual Database Tools