Vento Aureo - Chapter 135 
Polnareff explains that the soul switch is due to his Stand Chariot Requiem, who he discovered could transform from Silver Chariot if the Stand was pricked with the Arrow.
Polnareff explains that the soul switch is only one aspect of Chariot Requiem's power of soul manipulation. He also explains that due to his Stand being out of control, yet following the instruction of protecting the Arrow, the party will have to somehow defeat Chariot Requiem to use the Arrow on themselves.
Polnareff reveals that Diavolo was collaborating closely with someone else, which is met with incredulity but the discussion is cut short when Diavolo is seen rushing toward Chariot Requiem. The group runs to stop him but Diavolo then summons Sticky Fingers and cuts off Requiem's arm.