Adding custom attribute to old work items

If I add a custom attribute to a work item type, it will show up on old work items of that same type, but, I won't be able to choose anything -- it will just be a grayed out field. Is there any way to fix this?

Example: I add an enumeration to a Task. All new tasks will have that custom attribute available. All old tasks will show the combobox for the enumeration, but, there will be no text in there and it will be grayed out and disabled.