In a lot of ways, turning Mysterio heel would hurt them in the same way they fear turning Cena will hurt them. He sells a lot of merchandise, is hugely over with the kids, and is responsible for a large part of WWE's Hispanic audience. I guess they're banking on Sin Cara's stock skyrocketing to make up for the loss brought about by a Rey turn. Not to mention, one thing that's key in making an effective heel is an effective face to play off of. Sin Cara's mic skills are non-existant, I'm not sure that he speaks English at all, and a feud, especially a Wrestlemania feud, needs to have talking. Plus, after the feud is over, then what do they do with heel Rey, turn him back just like that? It could work with him saying he was blinded by jealousy or something, but really, what's the point of turning for one feud?
Plus if they're going for the World Record, it'll be easier to achieve if both men are faces. Just have them face off in a friendly competition, just to prove who's better or so Rey knows he can still go or something, maybe acknowledge that he's missed last year's Mania due to injury and wants 29 to be something special. Either that or a tag team match and put the tag titles on them that night, since the plan seems to be putting the belts on the Primetime Players and giving them a fairly lengthy reign.
Bottom line, Rey shouldn't be turned heel, and Cara definitely shouldn't be turned heel.