I don't think it has anything to do with the fact that Michael Corleone disliked Tom Hagen. I think it was to do with Michael wanting to be completely in control of the way the Corleone Family was ran. Also, Carlo Rizzi was never officially promoted to Consigliere. His position within the Corleone Family remained as a Soldier. We learn that he is "Made" when Don Corleone orders Tom to "give him a 'living,' but never discuss the Family business with him." I think you are referring to when Michael "promoted" him to his unofficial advisor, since he was from Las Vegas, but none of his plans for Carlo really mattered, since he was already sure that Carlo was the traitor, who set Sonny Corleone up for the Barzini/Tattaglia hit at the Causeway and he had probably already made plans to have Carlo murdered. Also, when Michael returned to Nevada from his business trips to Miami and Cuba, Tom took up his usual position as the Family lawyer, but he still held a high position within the Family.