This happens to me alot. When economy is completely full, they turn economy extra into regular economy. Usually, EBG and Star Alliance gold are automatically put in these seats, and you can move around if you want to, i myself an EBG flew on 943 on dec. 16th. I was put in 15B, i moved myself one row forward. And when I got to CPH, I was bumped up to 2C! When its this full, its C for EBGs 9 outta 10 times