The new Medicare Card program includes a transition from the HICN (which is prefixed by an SSN) to a new Medicare Beneficiary Identifier (MBI). However, the identifier registry has no mention of the MBI. So, please add a new row for the MBI. Note, it should be different from http://hl7.org/fhir/sid/us-medicare since it has a different OID: 2.16.840.1.113883.4.927. Also, be sure to update the documentation for http://hl7.org/fhir/sid/us-medicare to note that it does not encompass the MBI.