1. You cannot change seats at United.com. You have to call reservations.
2. The front several rows of each plane, called Economy Plus, have a few inches of extra leg room and are reserved for elites or people traveling on a full fare economy ticket (Y/B fare). You can view the seat maps for all of United's planes at www.seatguru.com and you'll find out which rows are designated Economy Plus. If you can manage to get a seat in one of these rows you will definitely like the seat better than regular economy (no other difference except extra leg room). But if you do get one it will likely be by the good graces of the gate agent when you check in, not in advance. The seats you see at the very back of the plane are not Economy Plus but they are usually blocked out up until a day or two before the flight I guess to give the airline more control at the very end. 1K's can view and select these seats anytime in advance but no one else can (and I can't imagine any 1K wanting them!)