Wirelessly posted (Mozilla/5.0 (BlackBerry; U; BlackBerry 9930; en-US) AppleWebKit/534.11+ (KHTML, like Gecko) Version/7.1.0.755 Mobile Safari/534.11+)
SQ will, IME, absolutely notice if the passenger in the biz seat is not the person assigned to that seat, particularly when they greet you by name upon boarding. That said, it should not be a problem at all to trade the seat; simply inform the FA of your intentions.