If the flight number or schedule changed, it's likely that the flight was requalified. As I understand it, all of the passenger records are processed for potential schedule conflicts or invalid seat assignments, such as when a new-config 777 is switched for an old-config bird. Even if the equipment (airplane type) stays the same, the process may cause seat assignments to be changed. This is normal, but still disconcerting if you have specific seat preferences.

Changes like this are pretty rare, but checking seat assignments is always a good idea.