Michael's giving Carlo a more important position was all part of the plan to lull the other Families into believing a weakened Corleone Family was keeping the peace. This also gave Carlo a false sense of security, believing he finally had a prominent place within the Family and was free from blame of Sonny's death.

I believe the announcement Michael made was for Carlo to be his 'right-hand' man once the move was made to Vegas. As you can see, Carlo never made it to Vegas.

Connie was absolutely correct when she hysterically proclaimed that Michael knew all along he was going to kill Carlo.

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