The usual line given by GAs (and even Intl BF concierge) whenever your seat assignments get changed is that the equipment has changed.

As I have learned, that is not always true. On one occassion, the day before our flight, a first Intl BF concierge cancelled my family's seat assignments in BF (IAH-CDG) to give our seats to another couple. We were not given any other seat assignments. At the airport, after 30 minutes of giving us (by a second Intl BF concierge) the standard GA BS line that we lost our seat assignments due to an equipment change, a third Intl BF finally fessed up and said that the first Intl BF concierge improperly cancelled our seat assignments.

However, I have to give kudos to the third Intl concierge, who literally contacted every passenger in the BF cabin to change people around so we could get seat assignments (row 4) close to where we should have been (row 3).