Change Flight Date before Check In

Hello to the community!

I made a reservation for 2 adults, with nothing extra: 6-2-2013 from Thessaloniki, Greece to Rome and 10-3-2013 from Rome to Thessaloniki. However, I made a mistake. I need the return to be on 10-2-2013 and not on 10-3-2013. I haven't checked in luckily too. I saw the tip here ryanair.com/en/terms-and-conditions#regulations-flightandnamechanges and the prices here ryanair.com/en/terms-and-conditions#regulations-tableoffees.

If I get it all correctly, I'm on the low season and on the normal flight, and I'll only have to pay 60 euro (30 for each person)? Any help will be much appreciated!

It seems like I made a mistake. I think that, additionaly, I'll have to pay the sum of the new flight minus the sum of the old one. Sadly, the new one costs almost 2 times most.