If I wanted to change an item's enchantment (say, switch it from Cast On to Constant Effect) do I simply just look up the item, find its enchantment, then go the enchantment tab and make my changes and then save as an esp? Or is there more to it?

Well, if you *change* an enchantment in the enchantment tab, you will change it for every item this enchantment might be used for. Correct is to create a new enchantment and assign it to the item.

Will try. Thanks!