Since all menus come with 4 courses, what you can do is choose between menus in the same course. For example you can choose course 1 in menu 1 and course 2 in menu 2, and course 3 in menu 3, and course 4 in menu 1. As long as the dish you want to switch to is in the same course you want to replace, there should be no problem. I have done it many times in the past so should be no problems.
They don't make substitutions. My family went in October and we shared off of each other's plates..
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