Set the field editor to edit mode, with a rendering rule of Case Status != Closed. Add a second field editor in read-only status and a rendering rule of Case Status = Closed.
We sometimes do this with individual fields in a single field editor. One occurrence of the field is rendered when a condition is true, and the other is rendered when the condition is false. As an example, if a certain field has a specific value or is not blank, another field appears to switch from optional to required.
