File plan reference Id

How do I reuse a file plan reference Id in Records Management? I have created a file plan reference Id but want to use it with a different label so I have removed it from the original label. All the fields on the Editing File plan descriptors screen are dropdown fields with values but the Reference Id is just a dropdown with the only option as "Add a new file plan descriptor reference Id." It doesn't allow me to enter anything in the field and if I try to add a new one that is the same as the one I created it gives me an error and says it already exists. Shouldn't the ones I have created and not used be in the dropdown? See attached for details.