During my IAD-OAK flight a couple of weeks ago, I was moved out of my chosen 8C for exactly this reason - a wheelchair passenger needed to be accomodated and the armrest on 8C could be raised, hence my seat was needed. After paging me to the podium, the GA explained all of this, put me in an exit row (window, but it was the one without a seat in front), and even gave me a $50 voucher "for the inconvenience." Now that's a good way to handle it - win-win for everyone (except maybe UA, since they're out $50).

Too bad that many (most?) GAs don't handle it this well.