In the last couple packs I have made, I had the need to replace a certain item in all recipes which it appeared in, and could not find an easy way to do it.

For instance... if I wanted all recipes which use leather, to use wool instead, I would have to manually recreate all of the recipes using wool. May I recommend a way to "swap" instances of an item within all recipes at once?

Something like:

recipes.replace(minecraft:leather, minecraft:wool);

You mean like this?
http://crafttweaker.readthedocs.io/en/latest/#Vanilla/Recipes/Crafting/Recipes_Crafting_Table#replace-all-item-ingredients-with-another-one

Yes! Man, you added that feature quick!

j/k thanks!