Replacing aircraft right before take off

I do understand the importance of the airline industry being flexible in responding to customer demand. When I ordered my San Francisco to Tokyo flight about 4 weeks ago (middle of November 2011) I selected my seat on a United 777-200 new configuration airplane. I selected a lovely isle seat in 21F Economy Plus cabin. Now the 777 airplanes don’t carry as many passengers as the larger double decks 747, and when United later realized they were selling out the 777 airplane, and even the larger 747, United conveniently, and quietly, changed airplanes from the 777 to the 747. This turned out to be a disaster for me as my lovely 21F isle seat suddenly became a horror 21F middle seat. You can see the seat configuration below and see how United treats their 1K members when just quietly swapping people from isle seats to middle seats. So the rule is, check your seat selection every day at United.com to make sure they don’t quietly replace your original selected seat.