Re: BF2 - change sides
@sjiveruOf course you can change sides.
I haven't played this mode since a very long time, but if I remember correctly, it's like in singleplayer arcade mode that you have to choose your side before starting the match in the match setup menu.
It's just not so clearly distinguished, like in Arcade it's probably some small button at the bottom of your screen that says something like "change faction" before you let the game load up the level.
I hope this helped.