The earliest you can do the $25 same-day confirmed change is 3 hours prior to departure. It just so happens that the last EUA sweep before it is turned over to the airport occurs right at the 3 hour mark. At that point you would be placed on the waitlist for an upgrade according to elite level and fare class but your check-in time would reflect time you were confirmed on the new flight so if it came down to that deciding factor you would be pretty low on the list.