Sounds like this was a last minute change. That happens, unfortunately, and with configurations changing, yeah, is a PITA, especially since they don't appear to properly manage how the seats convert from one configuration to another.

There's not much you can do, except to keep checking. There's a period up until about 2-3 months in advance (IIRC) where a change can happen, and then beyond that, it's usually mostly within 3 days of departure, as they assign specific ships to a flight, and then adjust as necessary for purposes of operating the network given any last minute maintenance issues, IRROPS and the like. Trust me, it's not really something they want to do either.

Think of it this way - would you prefer to have a canceled flight if the originally scheduled aircraft went MX the day of, leaving no plane to take you on, or a extremely late inbound leaving you with an hours delay, or a different sub-type that gets you in close to schedule. I'd suspect most people would want the latter.