You can now make changes 24 hours in advance and it doesn't have to be the same calendar day.
Note it has to be 24 hours from the flight you're trying to change to. This has a couple of interesting implications. For example, if you were booked on a 6:00 PM flight on Saturday and want change to a 2:00 PM flight that Saturday instead, you can make that change at 2:00 PM on Friday, even though it's more than 24 hours from your initial flight time. Same thing in reverse if you want to fly later, you get less than 24 hours from your original flight. Also, if you're looking to change a second or later flight in a multi-segment itinerary the 24 hours is from the specific flight you're trying to change, not your initial check-in flight time.