I think that Michael telling Hagen that he was out was all part of the major plan! Michael and Vito both new that eventually someone would try to kill Michael once Vito died. In essence there was absolutely no one that Michael would be able to trust, so keeping Hagen out of the overall plans would leave one person that Michael could eventually go to in the event that any attempt was made on his life. This became evident after Tessio approached Mike with the offer of a meeting. Then and only then did Michael tell Tom his plan. The Corleones also were making moves in Nevada to get into legitimate businesses, casino's etc., and Hagen would serve better as a legitimate lawyer to handle these affairs without ever being involved or tied to inside illegal family business.
This also became evident in GFII after they shoot up Michaels home! Who does Michael go to? Tom Hagen, the ONLY one that he could trust with his and his families life! Why? Because Tom was kept in the dark and therefore it would have been impossible for Tom to be the one on the inside to set up Mike.

Don Cardi