Anyone else ever have this happen in airplane seating?

Saturday before last, we flew from San Francisco to Auckland, New Zealand on a red-eye overnight flight. We had bought our tickets well in advance, and selected seats in the next-to-last row as indicated on the seat selection screen. We did this because the last two rows are only two seats across, and as it was just my daughter and I, we thought this would be great-- a little more room, and no having to get or wake the aisle person up if you have to go to the bathroom in the middle of the night.

We picked the next-to-last row (seats k and l), so we could recline our seats. Row 51 in this seating diagram:



In San Francisco, we boarded the plane, and I immediately made a beeline to the next-to-last row. I was about to sit down when my daughter touched me on the shoulder and said, "Dad, that's not our seats. Row 51 is up here a few rows". Damned if it wasn't true. The plane we were on had 5 more rows:



I was more than a little miffed; we had picked those seats for a reason, and as recently as a week before when I changed our domestic connections I made sure we had the same two-seat row-- and even SAW them on the seating diagram-- for that long international flight.

Looked it up after we landed, and apparently the difference is that our tickets were sold on the basis of United's Boing 777-200, but the plane they used was the 777-300ER. Two different variations of the same plane.

This ever happen to anyone else?

I can only assume the plane filled up and was fully booked, and in the last week or so they switched to the second variant so they could sell more seats. I remember an announcement before boarding saying they could seat all their standbys... kind of not fair to those who picked seats well in advance, however.