The only thing about a Cena heel turn: right now, there's a clear vulnerability, a clear reason to turn. Cena's always got some boos, but they never intentionally put Cena's popularity up against someone else's, not when Cena's lead is tenuous at best. Take a look at other huge heel turns:
Hulk Hogan. There was nothing left for his character to do as a face. Hogan got a lot of negative reactions in 95-96 after his big start in WCW. The people were quitting on Hogan, he had reason to turn.
Steve Austin. He had just lost a year to injury. And instead of dominating as he had before, he was going even with HHH, and the crowd was split between them. Again, there's the vulnerability that explains the turn.
Cena's vulnerable from a popularity standpoint, but not from a wrestling standpoint. If they really wanted to set up the turn, they set it up by having Cena lose clean a few times, give some sense that he can't go as well as he could before.