This thread raises a lot of questions (especially in light of
Lexus-gate). Is moving from a middle to a window poaching? What about a window to a middle? What about within one's row? Or section? What if the seats have no extra legroom but are closer to the front? I don't have any answers but clearly the distinction is not as clear as coach vs. first.
Historically, flight attendants would allow passengers to spread out within their "section." But with the advent of preferred seating, that is now impractical. There's no way to identify preferred seats without using the app AFAIK.