I definitely would not trade Casey for Pettersson or a similar defender. I wouldn’t even trade him for Chychrun either. Just not wise asset management.
Moving Casey would just be a short sighted overreaction to this year, in my opinion. Bahl, Holtz, Clarke, Foote, and picks should all have varying degrees of value to acquire a goaltender, middle-six forward, or a veteran defender. There’s some strong targets in free agency too for the forwards.
Just wait for Casey to develop and see where Luke/Nemec and the rest of the defense is at when he’s ready.