They should be less than F since they are not F. The whole point of YUP fares is that you pay less than F since you are buying Y and getting an upgrade at the time of booking. However if there is a change you are not guaranteed an upgrade seat on a different flight and many YUP fares and variants are not fully refundable or changeable at all. You must read the fare rules before booking .

Full fare Y and J and F where offered are fully refundable you get a seat as paid for fare. YUP are a deal on a sort of full fare Y. When you have full fare Y/J/F you can call AA and say change this flight to that as often as you like and they just change it and say anything else? No fees etc.