After deleting the rotate_secret operation, the remaining operations would be client_register and client_update. These take essentially the same parameters, except that the client_update operation would contain a client_id. The presence of the client_id parameter would be used to distinguish between these closely-related operations.
P.S. With these changes, “client delete” operation could be performed with an HTTP DELETE method and a “get client status” operation could be performed with a HTTP GET method. I’m not advocating these operations, because I don’t think they’re needed in a minimal spec, but I did want to put out that the simplifications proposed above would align with the possibility of adding them later, should the working group decide that it’s important to do so.