So when did the situation change from "some one close, some one who will propose brazini meeting, some one who is unhappy with mike taking over, etc. etc" to "Tessio is THE traitor"?

I guess it must have been the funeral, because the tapping of phone and Tessio expressing dissent, both happened before Vito died. But Vito did not know conclusively if the traitor would be tessio or clemenza or any one else.