BA automatically checking you in and emailing boarding pass
Why for for does having the boarding pass printed prevent you from changing seats at the airport? I'm not sure I've done it on BA but certainly I've changed seats at kiosks, checkin desks, and even gates on other airlines, never a problem.
the main problem I see is that there won't be many seats left if they've checked everyone in already. In fact surely the last minute business purchasers must be crying bloody murder over this.