Flying IAH - LAX earlier this week (booked 2 weeks ago)
I'm in 9C, PAX in 9A asks me if I paid for my upgrade to E+. I said, "no, it's complimentary at booking because of status." He goes on to tell me that he selected 9C at booking a month ago, and his seat only changed after he got to the airport. He showed me his paper (printed from home) boarding pass showing 9C, and the email from when he purchased the seat also showing 9C. His current electronic boarding pass had 9A.
Seemed odd, and I offered him the aisle because he was very nice about the whole thing, but he said he was fine with the window, just curious why he got moved.