I, too, was baffled by this additional bag fee. Original flight included a carry-on bag for £10, and the flight I was looking to change to quoted me £10. Since these are identical amounts, it really seemed that Ryanair were saying the original fee was something I would lose, and I'd have to pay a second time. It would never have occurred to me that a "free date change" due to covid would entail us being hit with post-booking fees for the extras! When I look more closely at the check-in luggage, however, I can see that the explanation provided by MB_again is correct: for this one I originally paid £22.99, and the new fee on the changed booking is £42.99, and the system wants to charge me £20, which is obviously the difference between the two.

BUT IT DOESN'T END THERE! Try changing a child's flight and the fare difference they expect you to pay for a child's flight has the £13 APD discount added back on!