this is such a ........ airline.
i too tried to change a reservation that included bags. there is 0 fee for the flight, but they do want a price difference for the bags.. between the price when you purchase the bags with the reservation , and after the reservation !
this is ridiculous !

and starting 15/6 there is a much higher price for bags, so changing to a date after 15/6 causes a ridiculous price !

so everyone beware of this "zero change" policy.