I have done this a few times,
in “My Trips”-> “manage my booking” -you need to be logged in, the system knows you have a fully paid booking, and knows it is only any applicable difference, is to be charged, i did not have any issues.
The system asks you for your card details for any excess if applicable, as people change cards, etc, so system does not assume, that the card you used to make original booking, is the one you want to use for the flight change. There is also, baggage changes, which could add additional cost, and the Aer Lingus carry on policy changed a month or so ago, so you now have to pay extra to bring a carry on travel bag onboard.
I experienced zero issues anyway, but i have also got through(eventually) to an agent by phone, to change something or other, i could not do on manage my booking.