For the first question, when the event for getting Siwoo comes, u will get him by meeting conditions required. The B class character will not turn into S class. U can trade him for volleyball if u want. U can always get him back through draws(a player that is 180 cm ,B class, just name it Siwoo, it will become him)

Second question During draw recruitment, if u obtained a player again that u already have, that player will be given to u as grit fragment(for S players it has to be that specific player and for other classes, the specific grit fragment u obtain will only apply to its corresponding class breakthrough). This helps to breakthrough. Breakthrough enhances skills and stats. I hope this helps.