Every time you change your ticket, you will be subject to paying any increase in fare difference. In the future, you will also probably have to pay the change fee (which is currently waived).
If you move to a "cheaper" date, you don't get a "refund". You cannot therefore expect to get that "refund", even in the form of a "credit", at any further point in the future if/when you then make further changes to a more expensive date.
You therefore gain absolutely nothing by changing the ticket now to a date that you fully intend not to travel on. You can currently change the ticket for travel out to Feb or maybe even Mar 2021 on most routes. Select a date sufficiently distant where you can commit to travelling (and, in that distant timeframe if it turns out you can't travel as re-booked, then change the ticket for a second time - which brings you to the same scenario that you were actively considering doing now anyway).
(But bear in mind that there is now at least a risk that Aer Lingus may not survive the current crisis)