I wouldn't worry about it too much as I've had this happen to me once before as well. I had a great EC seat (second row, window) and was switched to non EC middle.

Went to talk to the AC agent, and the best he could figure out was that the GA moved my seat to accommodate a family, knowing that I was number one on the UG list and I would most likely get a FC seat.

I still didn't like it because it seemed a little risky to me. What if the previous flight canceled and FC filled up. What if a last minute purchase bought out the remaining 2 seats. But everything worked out in the end so I didn't complain.

Do you know where you stand on the UG list? Are there still seats available up front? If not, I would definitely talk to the TA, GA, or AC about it once you get to the airport.