I'll have to disagree w/ most on this topic. If someone's paid for 1st class, he/she's paid for 1st class. Someone getting upgraded is occupying a seat he/she did not pay for. If you're bumped back, CO is still fulfilling the terms of their contract w/ you, and with the 1st class fare bumping you back. If you are not bumped back, CO will not be able to fulfill it's contract w/ the paying 1st class customer. They would prefer to fulfill both contracts for obvious reasons. It's unfortunate there's a rare bump back, but it wasn't your seat anyway.

I once was on a plane from Boston connecting to houston on a 1st class fare. The plane out of Boston was late due to CO mechanical problem. The EWK gate staff (thinking we weren't gonna make it) upgraded some economy passengers. We ended making it and two people who had temporarily taken our seats were escorted back (of course complaining). Do you really expect CO to leave the upgraded cheap fare persons up front and put the 1st class fare people out back? Think real hard (using CO's perspective, not "what you want" perspective). The logic is pretty straight-forward. If I was bumped back, I'd be fairly understanding.