I'll just be bold, I don't think the WWE is ready to turn Cena into a heel for the same reasons I believed he would win at HIAC. Cena sells too merchandise for WWE to go with a good storyline involving a heel turn. Here's the ways I foresee this ending at Survivor Series.
1) Cena will deny Barrett the Championship, get fired, and just take some time off.
2) Cena will give Barrett the title, immediately turn on him, get some heat but still run around doing the same thing he's been doing all year long until everyone forgets about it.
3) Cena will give Barrett the title, and Nexus will beat him up, and he'll take some time off.