It pains me to say that Apple is dead on.

Fredo didn't have a clue. He really believed that Michael had forgiven him, and he was obviously put out by the fact that Anthony could not go. The key element in the "change of plans" thing in all three GF movies is that someone is suddenly moving out of danger unexpectedly.
Thus Fredo in GF II didn't know any more than Carlo knew he was about to be garotted in GF I.
Likewise in GF III I think Mike first realized that Altobello was a traitor when he conveniently "escused himself" from the meeting room seconds before the helicopter hit.