What happened to channa is the reason that one should always immediately change a seat on a 757 if it is row 5 or 6 to a seat in the first 3-4 rows (despite what the seatguru says about row 6). If there is an equipment change, you are generally out of luck with an EUA'd seat in the last two rows.
This is especially galling when the last minute switch is to a 757 with international configuration where you not only lose being upfront, you also lose the promotion to the BF seat.