Sharon, I have also booked with United, and had one or more flights operated by Lufthansa, sometimes trans-Atlantic and sometimes within Europe, and paid and on mileage. I found the same thing, that I could not select a seat on Lufthansa until 23 hours before departure, even with a Lufthansa confirmation number, and even if I was willing to pay for "extra legroom". I have spoken with Lufthansa customer service people various times. One time I spoke with a United customer service rep, and he explained the issue for me. When United and Lufthansa began operating joint flights on Lufthansa planes, this became an issue. Lufthansa charged for their passengers being able to select their seats, and they wanted to charge United customers also. United did not charge their customers for seat selection, and did not want to do so for these flights either. So, this was the compromise, Lufthansa would not charge United customers for seat selection, but would allow selection only 23 hours before departure. This appears to be a constant problem, because United customers, even when booking months ahead, can not avoid middle seats, cannot arrange for extra legroom either paid or unpaid, and, I imagine, a family can not assure they can sit together on a crowded flight. This is not stated when you are booking your ticket. Doesn't make sense to me. I myself am making a series of flights booked on United next week, and fortunately only Frankfurt to Sofia is on Lufthansa, and for that particular flight segment I have no seat assignment (and will be over the Atlantic 23 hours before departure).
