If an Elite (presently Silver) books a full fare Y roundtrip ticket, say from AUS to SEA (via IAH) departing on the last flight on a Friday evening and returning on the last flight on Sunday evening (i.e. red-eye for arrival into AUS Monday morning), -AND- they perform the upgrade-from-Y-to-first during booking, then what is the likelihood that they would retain their first class seatings for the duration of the trip? Fairly iron-clad, or not worth risking the $200 difference in fare by purchasing an A class ticket?

thanks much!