I actually covered this before in another thread. It would be easy and effective to do it right now.
Some have stated that he is such a huge face that he should stay that way, others that he is in the public eye too much and so on, he's the big marketing machine and everything, I know. But I mentioned before that the face thing for him is so old now he really needs a change. If you keep him as a face for too long people will start to loose interest and they already have. It's horrible, the boo's are almost louder than the pops, and what about when ever he gets into a slug fest with someone the " Boo, Yeah, Boo, Yeah" Chant that starts every time now. That is obvious enough I think. He won for so long that it got boring just watching him win all the time. It got to the point you didn't even want to watch because you knew he'd win every time but still did in the hopes that someone might beat him, and for God sake it took him getting injured to the point that he couldn't wrestle before he really did.
Now though, we have seen him loose, and the fans have got a little redemption from the last 3 years or so of Cena domination and saturation. He has lost been injured, re-injured, and beaten conclusively a number of times in a row and now he has a reason to turn.
He could come out and say how everyone boo's him even though he comes out and works his ass off for them, and how once he got injured everyone forgot about him, and he want's his title back, and how being the stand up good guy never got him anything but a bunch of snot nosed kids asking him for his autograph so they could go sell it on eBay, it would be prefect and make peoples heads spin.
The Cena fans would be appalled and all the kids would cry and their little hearts would be broken, and the people who already hated him could hate him more drawing ever more heat, and then some of the people who hated him would like him since he was a heel. All around he still stays one of the biggest stars in the company and it couldn't come at a better time that now because he at the point now that his gimmick is getting stagnant and stale. Triple H has proven over time that you have to reinvest your character, Undertaker has proven the same ,and we have seen more recently Chris Jericho do it and it worked nicely for him. Now that some stuff has happened to Cena, namely him recent loosing streak, and injuries you could make it believable that he has some hate in him, and that he's pissed off about things, it could work so perfectly.