This just happened to me a couple of weeks ago. Everyone above is correct.

However, if by chance you don't like any of the F choices that day, I'm pretty sure you can say "can I have my preordered whatever from Y instead" and you'll get it for free. On my flight I could see that at least one person got the cheese tray and one got a snack pack rather than the F meal.

Edit: cheese tray was one of the F choices, but the person across from me definitely said she didn't like any of the F choices and got a snack pack.