My main gripe is that, same day flight change is supposed to offer flexibility, but if it's fare class restrictive, it's kinda oxymoronic. Obviously there are less seat inventory come the day of departure and obviously the earlier booked cheaper fares are more often sold out other than full Y. This sort of defeats the whole purpose of SDC flexibility. Anyway, gotta give it to Delta for being so shrewd, I guess that's why they're more profitable.