If the seat if filled, it usually means someone has chosen it. Sometimes seats are blocked for crew rest or other reasons, but that seat isn't going to be sold. So, in my experience, a seat filled is a seat that won't be sold to someone. OTOH, a lot of open seats means nothing. Seats may have been sold and folks just haven't chosen a seat yet.

If someone is upgraded, then the seat will open up, but that's usually fairly close to the day of departure (depending on airline, etc.)