Consider the work entailed over just changing the skintone codes. Personally, I think this method is too much work - in some skin sets (and your original one does), each skin will have textures for each gender and age group and you'll have to replace them all. If you try to overwrite the textures of the old skin with the new ones, they'll look like Lilith's skin but several other properties like genetic values will still be those of the old skins. As far as I know, SimPE is the easiest way to do change multiple sims' eye and skintone values. You'll have to find the codes for your new skintones and use them in the tutorial, but they'll use the same genetic values you've (or the creator) set for them. Here's a tutorial on changing sims' eyes and skintones.
