I think this is just slightly clumsy language on the website - you can request a flight change any time the day before your flight, ie, within 24 hours of the date of travel rather than within 24 hours of the time of travel

I was once offered a change to the day before FOC under that benefit, but it seemed to me the agent wasn't reading the rules correctly (I could hear him reading the words off the website) rather than that being what they intended the benefit to be. I get the impression that all the Concierge benefits at the call centre are just granted through remembering the rules and just manually calculating what the charge should be rather than the booking system doing the work for them.