There are several different changes that get lumped into a 'version', but each needs different handling. In each case, appropriate mechanisms already exist without adding a version. The call DTD changes. The reply DTD changes. Use of new reply DTDs should be requested by appropriate call headers.
Clients require new server capabilities.
Clients can take advantage of new server capabilities. The defined fields are code , message , and detail . In Java, detail will be a serialized exception. The SML-RPC request has some sort of syntactic error. The requested object does not exist.