this is wise, thank you!. i guess i'd try calling after midnight to let them know i will be missing the 7 am flight (and to keep my itin as i intend to use it at 6pm...), then re-calling at noon to confirm the flight. fun times!

per agent conversation, any open seat in coach is sufficient to same-day-change. no need for specific fare class. this is good news as the desired flight is selling quickly