Another case of Common Sense not being applied. Case in point. You book front row seats which also happen to be bassinet seats. Some airlines do not allow those seats to be booked until closer to the departure date or make it clear that you might be subject to seat reallocation in the event a bassinet is booked. So a simple notice in the booking system which states that “the extra leg room booking and extra payment is subject to the traveler being able to carry out the required duties, see below or wherever, so be sure you can before you complete the booking. You will be subject to reallocation if it becomes apparent that you cannot carry out said duties” – or words to that affect. From the comments above I can only deduce that not to move a passenger/s who are incapable of carrying out the required duties would be in breach of aviation law. Know doubt someone here will correct me if I am wrong.