To be fair, if it still exists, ez do allow some flexibility, but with strings. On the return sector you can catch an earlier flight on the same day space available. I discovered the 'strings' when I turned up to avail myself of this excellent offer. Then there was a Duh moment. My wife had driven down on holiday as I was on duty flying. I then booked a single ticket to meet up and was available earlier than expected. I turned up to catch the earlier flight. No can do; it was not a return sector. Duh! Why make an easy job difficult? If they can do it on a return sector why not the outbound or a single sector as well? After all, there are separate tickets/prices for each sector; it is only the booking reference that attaches to both.
How does it work now? It was a very attractive benefit versus competitors and costs them nothing. Indeed they then have an empty seat for last minute bookings at high price. So why not do the job properly?