Agree with highflyer.
In effect they exchange your old ticket and issue a new one in the next available fare bracket and as a goodwill gesture they put the base fare of your original ticket towards your new one (fare difference).
If your original fare was £50.00 and the new fare is £90 then you pay the £40 difference. If your original fare was £50.00 and your new fare is £52.00 then you pay £2.00 difference.
And then the majority of airlines also charge a change fee or admin fee per person per sector, which varies between airlines.
Obviously if you have an all singing all dancing ticket then you can usually change it unlimited times free of charge providing there isn't a name change or route change (to a more expensive route).