It's been said before but it's worth repeating: if the phone agent is giving you the wrong answer, call back. After being informed yesterday that it would cost $3000

to change the date of my upgraded WBC LAX-LON trip home for Christmas, today I got the sensible answer that it costs $250. So I get another day at home, and I get upgraded on all six legs instead of 5/6 as before.
I am now a happy Xmas bunny