The request to add replaceBy back into the resource is rejected because: a. From both design and implementation perspectives, it's preferable for the history of a Naming System (and each Unique ID used by that Naming System) to be held within a single instance of a Naming System resource and the elements needed to achieve that goal are in place (i.e. uniqueId.type and uniqueId.period).   b. As a Reference to one, or many, other NamingSystem resources, the replacedBy element could be used to create circular references. Instead, other FHIR mechanisms can be used to achieve the desired result: The standard FHIR versioning mechanism allows the resource (same 'id') to be updated with the new information (and the prior history can be retrieved as needed). The Provenance resource can be used to provide additional data about the change.