This is unfortunate, but maybe you can take a little solace that you are not alone. I just had a cabin configuration change (old 789 to new) and my brother was moved to the back of the plane in a middle seat. Called United and we won't be able to sit near each other (the plane is nearly full), but at least he now has a window seat. It is frustrating.

I think the only thing you can do is to frequently look at the seat maps and hope that better seats will become available.

I've had flight changes on AA but those have happened 6-8 weeks in advance, nothing close to departure.