Same thing happened during the time period when they were transitioning the Intl 772s to Polaris (old to new; back to old). We ended up getting split up when they switched back to the old configuration at around T-24 hr, and by that time the cabin was booked full. Even if traveling solo, no guarantee that you get your originally assigned seat back.

Right now, I'm trying to avoid these right now if there's another lie-flat option.