Issue Description:

I want to make a function that receives three items and changes every occurrence of the second one with the third in the first's recipe.

function replaceInRecipe(recipe as IItemStack, from as IItemStack, to as IItemStack){}
is that possible in any way? it would make my life so much easier

You could try using recipes.getRecipesFor(IIngredient output);
That returns a List of IRecipe objects.
These objects have some getters to receive the used ingredients.
Heres a wiki entry for them
http://crafttwaeker-docu.readthedocs.io/en/latest/#Vanilla/Recipes/Crafting/ICraftingRecipe/

Also, note there's a PR currently that might add the functionality to replace certain ingredients in the future.

Thank you, that seems like exactly what i need. I'm gonna keep an eye out for that pr as well

It is, but what is not yet implemented is casting a List to an Array. Try removing as IRecipe[].
In a later CrT release lists will have the identifier [baseType] but that is not yet released

Edit:
Also, it's called ICraftingRecipe not IRecipe (and you need to import the class, of course)

It seems it's not implemented yet

function replaceInRecipe(recipe as IItemStack, from as IItemStack, to as IItemStack){
  val inputRecipes = recipes.getRecipesFor(recipe) as IRecipe[];
  for inRec in inputRecipes{
  }
}

results in [CHAT] ERROR: [crafttweaker]: Error executing {[99:crafttweaker]: functions.zs}: Not yet implemented