I've found on the routes I fly that it is rare that a flight will be completely 0'd out and sold out, so I've readily been able to make same day changes. I do fly BOS MCO on occcasion, and if I had to do a same day change on that, I would not even lose a minute of sleep unless perhaps it was school vacation week, but still there would be open seats, there are so many flights a day on that route. If a flight is sold out, I typically set up an alert for 1 Y seat using ExpertFlyer and I would say 95% or more of the time a seat opens up. Again, though, its rare that a flight would be completely 0'd out on my routes in the first place.

Also the airport can readily do same day changes for Mosaic elites. If your airport agent is having trouble or they don't know how to do it, just tell them to reissue the ticket under irregular operations/involuntary reroute which waives all change fees and fare differences, or tell them to call crew support which is their internal help desk.

Also for anybody that knows there Sabre interface, I'm baffled why they always do a reissue for a simple same day change rather than doing a ticket revalidation. Have they blocked the ticket revalidation function from the interface that agents have?

Finally you can now in most cases do a same day change online or through the mobile app, and if you are Mosaic, they have the automation in place that will waive the fee & any fare difference when you do the online same day change.