Ford is probably sitting on spare plastic pans, and / or a commitment to source these pans for warranty work form whomever the current supplier is. Every ’17-’18 that came stock with plastic pans, will get plastic pans in warranty replacement efforts, until or unless the current plastic pan inventory is exhausted.
Additionally, since more than just the pan is implicated in this swap, they’re not just going to cough up the other parts out of the goodness of their hearts. They will if it becomes enough of a financial burden, but it’s probably still less than 2% of the model line affected.
Unless this becomes a financial headache for Ford on a scale that makes it more financially sound to obsolete the old pan and replace with the newer model “parts kit” this is how it’s going to be.