At the moment every Diet has allocated recipes - that means that if you change a Diet you will not see the same recipes as in previous Diets.

A hack for solving this is to save any recipe of your choice to Favorites when you are on a Diet - then you will still have access to the content through Favorites when changing to another Diet.

When it comes to a Meal Plan that is, unfortunately, not possible at the moment. You cannot save any Meal Plan recipe to Favorites due to some technical limitations.