I still say the only way to turn him would be against Taker at WM30. After 20 minutes of trying everything in the book and Taker kicking out, Cena ends up winning via some type of shady finish (chair, taser, brass knucks, etc.) The fans that already dislike him would....ok, nothing would change except they might dislike him even more, and the 13 and under Cena fans would turn on him because he was their hero who resorted to cheating to break "The Streak". Add in some backstage segments with a few fan favorites like Bryan and Punk, and it would only add fuel to the fire.
That said, I'm sure the story itself would be more complicated than that and the odds of it happening are similar to being hit by a bolt of lightning twice in the same day.