Ryanair and flight change for December

Unfortunately we had to change our Disneyland booking and as a result our flights were also effected as we booked them separately. There are 4 of us going in December. I tried to change the flight but it cost more to change it than booking a new flight.

I would assume that Ryanair would want to use the 4 seats that I won't be using in December so why charge me more to alter the flights. If I book new flights they wont be aware that 4 flights will be still available so wouldn't they be losing out in a way?

Is there any way I can get the flights altered at a cheaper price as the trip is not for another 6 months?