That effectively gives you $400 in fee credits in your first card membership year.
While most people use the credit to purchase 2 gift cards of $100 each from AA or United in separate orders, DDF user PlatinumGuy has his own method to get a straight cash refund that works perfectly though I’m not about to endorse it. You can read the Wiki on this DDF thread to learn more, though you need to be logged into DDF to read Wikis.
The language on the fee credit now says that “Card members who have already selected a qualifying airline will be able to change their choice one time each year in January at www.americanexpress.com/airlinechoice or by calling the number on the back of the Card.”
My guess is that when you call in after today that they will still let you change the airline, though why risk it?
If you open a new Platinum card you are not bound by that January time limit.