What I'm reading from that scene is that the idea of selecting a new codename was Kitty's idea, but the actual name choice was Rachel's. I think others are taking that comment too literally. There is no way Ray would let Kitty pick her new name, unless they were bunking together now (which isn't happening). Also I think Kitty has decided to use codenames in the field only, hence her catching herself and calling Rachel "Prestige".