In my opinion, it isn't. Not that I have an issue with Orton winning but Cena is hated by half the audience and Orton is one of the most beloved on the show and so you'd only get deafening cheers if Orton cashed in on Cena.
The best way to turn Orton heel would be to cash in on another hot baby face like Daniel Bryan. Imagine if Bryan was given a title shot against Cena, not because he won the briefcase or because he wins some random tournament or #1 contender's match, but rather because he's been on a roll lately with victories over Sheamus and Orton and The Shield and even looking like he had the match won last night before Axel interfered. He wins the title, then Orton cashes in. That's one way to do it.