Originally Posted by
ethernal
Not to say this is right or wrong, but on a 739 (which I believe this plane was), 10B/C is blocked for gate assignment (reserved for pax with disabilities). So the passenger that was originally in 10C was either upgraded same day or did not have a seat assignment (non-rev, standby, or basic economy). Since OP did not see them clear on the upgrade list, they would have to be either non-rev or basic economy (or flying on standby).
This makes sense, because if it isn't the case, it seems like there's a much better solution: if 17B was empty, why not move 17C up to 10B (giving 17C an upgrade) while putting the family in 17B+17C?