Send them a note.

I got involuntarily downgraded on a CPU a couple of weeks ago after I'd already taken my F seat. An F passenger showed up without a seat assignment and I'd snagged the last upgrade so I got bumped back.

It wasn't that big a deal because it was a short flight. I sent a short email to Customer Service and got a nice apology and a $200 travel voucher for my inconvenience. I was more than satisfied. What happened to you was way more annoying because of the length of the trip, so I'd expect something a little more than what I got.