Modify existing work item after customization

Let's assume I modify the Story type work item by adding a new attribute.  After adding the new attribute, I can verify my customization by creating a new Story work item, and enter a value for the new attribute I added.  That's fine for new work items.  My question is how do I enter a value for the new attribute for existing Story work items.  When I open an existing Story work item to edit, the new attribute field is not present to enter a value.  How do I enter a value for the new attribute in Story work items that existed before I added the new attribute to the Story type work item?