Unless US is different than other airlines, if your seat exists on the new plane you get to keep it - otherwise, you're SOL (so if you're in row 25 on a 320 that gets replaced by a 319 your row no longer exists and you get bumped.)
I have seen it happen where the new plane was larger and row 4 who would have been the first row in coach are now the last row in F. That was how America West did it.... so the last row in F might have been the passengers who originally had been assigned to Row 3 in coach....
I have been told that's why AC starts all their coach seating at Row 12 (except for their 777s and planes with no F) so if they have a plane swap it's easier to have passengers get on the new plane without having coach passengers unintentionally bumped up to F.