I am in a similar situation with this terrible scummy airline.

Already paid for bags for a flight i now need to change again as they will fly next week an empty plane to TFS so they dont have to refund passengers.

I called them up and was told i am being faced with a higher bag fee difference when i go to change the booking because i am no longer entitled to the 'online bag price' if i change my booking and i now have to pay the full bag fee on the revised flights. This means i have to pay the difference between the 30 pounds i have already paid and the 60 pounds im face with now. A new booking would pay 40 pounds for a bag so in my opinion at most i should pay 10 pounds more per bag per flight but alas they want to take the piss and charge 60