I'm within my 30-day preorder window, and it doesn't look like you can make substitutions when you pre-order. The sandwich has checkboxes that allow you to leave off the brie or bacon, but the other entrees I clicked on (including Gaston's Feast) had no ability to make changes. From what I've read, when you arrive at BoG you confirm your pre-order with a CM. If you're willing to chance not getting the substitution, you could ask then if they'll do it for you.