The thing is, it isn't a heel turn, this isn't a "turn to the darkside" type deal, this is a fundamental difference is personal philosophy. Sonora is the only one who really gets where Yura is coming from now, having seeing basically the same thing happen with Rin and that is Yura does not know how to wind back and relax and have fun with the game. Conversely, and the rest of the club need to called out on this, they do not understand her, they do not understand her drive, they don't not understand that cheating to win has left her with no honor and might as was worse to her then a loss and they churlishly dismiss all of that. Moreover, the reason, the biggest reason is Yura feels accomplished at C3, indeed she now feels that C3 is holding her back and wishes to be on Team Rin because she sees that Rin holds herself to a higher standard and discipline which she also desires. The point I am going for her is her defection to Team Rin feels like such a natural and logical piece of her chracter development and the rift between her and the rest of C3 is only going to grow now, that I would say that any sort of rejoining the team without proper reconciliation would be very cheap and lazy, and I do not put it past them to do something like that.