I've booked the following itinerary with leisurely 2h25 in ATH and 2h15 in SKG:

LGW-ATH - A3607 - 12:15-17:30
ATH-SKG - A3126 - 19:55-20:50 (Earlier flight: A3122 - 18:05-19:00, 3xC seats remaining)
SKG-ATH - A3129 - 23:05-23:55 (Earlier flight: A3127 - 21:35-22:30, 3xC seats remaining)

This is ticketed in full-fare business class which includes this note:
> Change to an earlier or later flight within the same day of departure, without a fee or fare difference charge. The service is currently available through web & mobile check-in (Aegean App or mobile browser) or at the airport (kiosks & counters).

If I arrive in time, I plan to see if there's availability to fly on the earlier flights. Unless my first flight arrives early, immigration will probably delay me too much to make the earlier ATH-SKG flight. But I think I may have a chance for the earlier SKG-ATH flight - unless all the seats are gone by then (which looks likely given the peak-time flight on this route).

I'm interested to hear any experiences of switching to an earlier flight using this feature of the full-fare classes. Is it a straight-forward procedure at the gate?