The draft specification states that the Bytes 1 to 9 of the variable header section of the CONNECT packet are currently a spec encoded string (2 prefix bytes indicating the string length followed by the string data) containing the letters 'MQIsdp' followed by the number 3. Changing these fields would be an easy way to differentiate between old and OASIS spec compliant clients.
Reasons for wanting to differentiate between clients include using discrimination of access to MQTT servers and understanding/implementing behavioural differences.