I think that turning him would be OK, but it would really hurt financially. If it were me, I would do something like having Cena win the Syxx-Pack Challenge at NOC in a hard tough fight, where he gets real beat up bad and the Miz comes in attacks Cena with the briefcase, and wins the title. Cena, due to his "injuries" would be out for about 2 to 3 months, so that they could pretty much kill off the "Super" Cena, and when he comes back, he is still a face, but with a fierce attitude into just destroying The Miz until he is hurt like he was. I think also a feud with a heel Triple H, when he comes back can bring back energy for Cena. Destroy SuperCena, and make him more into a mad, looking for revenge kind of guy that will do anything to get his vengence. Then, after all that, screw everything and then swerve them into a heel turn that nobody will see coming.