When you booked online you were offered the choice of accepting the 'lowest fare' option or 'flexible conditions'. ('Lowest fare' is pre-selected as the default option). There's a 'What's this?' link, which provides the following explanation:
"Lowest: The lowest fare available on the flights you selected. Changes and refunds to these fares may not be permitted or may be permitted upon payment of a penalty fee and possible increase in fare.
Flexible: The fare available on the flights you selected that permits changes and refunds with no penalty fees."
If you select the 'lowest fare' option you'll typically be offered a fare of �40 (excluding taxes and charges) for a one-way flight, booked well in advance, on that route. When you get to the 'Price quote' page, this information is clearly displayed:
"Time/date changes permitted at any time except on days of travel for a change fee of � 50 or an upgrade fee of � 50 plus any difference in fare. Changes subject to availability. Fees apply per ticket".
So you've either paid two lots of �50 for a return flight or, if it's a single flight, you've been charged �50 for the time change plus a further �50 because the flight you're now booked on (at today's price) costs �50 more than your original flight did (at the time when you booked it).
Those charges are standard practice, within the airline industry, for changes made to 'economy' tickets. (Except that some airlines don't allow any changes to economy tickets. If you want to change anything you have to tear up your existing tickets and make a fresh booking).
Only 'business' and 'first class' tickets only offer free flight changes. If you'd have clicked on 'flexible conditions' you'd have been offered a 'business' ticket, but at a typical cost (excluding taxes and charges) of �205, instead of �40.
Chris