This is exactly why I keep an eagle eye on my selected seats. I fly United (and sometimes Lufthansa as it is a Star Alliance partner). United will re-shuffle seat assignments using their computer... I have never figured out WHY they need to do this! I am a 1K with UA, and I have complained about my seats being changed without my approval. I usually can reclaim the seat I had selected, but not always. Just keep an eye on that seat assignment! If there are slight schedule changes, or equipment changes in particular, you can get moved around.

I've noticed that Lufthansa has been juggling around their equipment as they get new aircraft deliveries, especially the new A380. That lets them reposition A340, A330, and 747's to other routes, and not all of those planes are configured the same.

So it may be as simple as they change planes and reassigned seats from scratch.

Or maybe they just knew how much you love middle seats and just assumed you forgot to book one in the first place.

happens all the time. Not just for equipment changes. One really isn't guaranteed a specific seat -- even if you choose it months in advance.

Heck -- on some airlines you can't even choose a seat unless you have status.

We flew with Lufthansa in Oct. I made our seat requests very early, and reconfirmed them from time to time. A month or two prior to flight I noticed we had been changed. I called Lufthansa and asked why. A different plane had been assigned and we had been moved. I said I want the seats we requested originally, and they complied. I guess I was lucky that I noticed the change before it was too late to change back.

Thank you all for your replies. As best I can tell this is the same type of plane it was when I originally selected my seats. It sounds like this is a more common problem than I thought, maybe I've been lucky in the past not to have had this problem. I will keep an eye on my reservation and hope they will leave my seat assignment alone.