Elvis should have fired Parker around 1964 or disagreed a lot more with him.
It is rare in showbusiness for actors, singers or comedian's to stay with one agent throughout their careers.
If you read the Hollywood trade papers it is constantly being reported that some actor has just fired his agent and is changing representation.
Often times the agent is doing a decent job but the actors or singers still fire them and get someone else.
For the Colonel Parker/Elvis partnership 8-9 years was enough.
If Elvis hadn't of died Colonel Parker probably would have still managed him for 2 or 3 more years.