Yes they would give any difference in money back to you. But never change an F booking to Y. Booking Y with miles is a waste really and its also a big waste in my opinion to book just miles when BMI have suuch a generous miles+cash option. If you think about it.. If you end up paying miles £200 + £100 taxes in F then that is £300. If you booked just miles in Y that is same miles + £100 taxes. Thats means only £200 more to be in F!! And when you consider that that Y ticket would be about £300 and the same in F would be about £3000 then it is deffinately worth travelling in F!