Cangrejo caparazón de cromo

The creatures can’t attack until you start a turn with them under your control. So you can’t swap and then attack.

The exchange fails if either target is not still on the battlefield at that time.

The exchange fails if either you no longer control your creature or the other creature is now controlled by you.

The trigger occurs when you use the Morph ability to turn the card face up, or when an effect turns it face up. It will not trigger on being revealed or on leaving the battlefield.

You choose the two target creatures when the triggered ability is put onto the stack. You do not have to choose Chromeshell Crab. You choose whether or not to do the exchange on resolution.