I'm currently using these skins, there's like 24, ranging from light to dark, it has it's own genetics system too.
http://modthesims.info/d/535350

I've been using this skin for generations upon generations but I want to replace it now because I've fallen in love with Lilith's skins.
The problem is how complex the genetics I've been using now is, so I do not think replacing the skins can be a simple task

If there's no way to simply replace them, then I was thinking maybe I could edit and replace the files in SimPE by just overwriting the textures? But then keep the genetics so I don't muddle things up with my current sims.
I'm kind of unfamiliar with SimPE so I don't really know how to do that though, I don't know how default replacements work either

Please help me

I'm not clear what the problem is here? You can replace the defaults with new defaults without that being an issue. If you want to remove skins in use that are genetified, that's a more complicated job though it is doable.

Regular default skins (those replacing the S1-S4 skins) can be replaced without any issues.

Geneticized skins are not defaults - but they are custom skins that technically behave as defaults, meaning they have a genetic value ranging beween the S1-S4 skins. They're usually also towniefied, meaning newly generated townies can get the skins.

You should be able to remove the skins marked "defaults" and replace them with another set of defaults. However, you shouldn't remove the geneticized custom skins, because this can bork up genetics. If you don't want the skins to mix with the new defaults, you can disable the geneticized/towniefied ones by following the tutorial under "about genetics" in the link you posted. Then you can either stop using the (now custom) skins, or you can assign new skins to the sims with SimPE.

Your question seems to be about more than default skins. Defaults you would have 4 of, they replace the 4 maxis skins. Those you simply pull and change whenever you want. Default anything is easy.

Custom and geneticised skins are a different matter.

Right, I didn't know the exact terminology, it's mostly the custom and geneticised skins which are my problem

so in other words there's no easy way to replace one custom skin with another so that all sims using that skin will use the other?

@simmer22 if I replace my sims skin in SimPE will the genetics change too? If I remove the custom skins in what way will it bork up my genetics?

Here's a tutorial on changing sims' eyes and skintones. 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. As far as I know, SimPE is the easiest way to do change multiple sims' eye and skintone values.

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. 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. Consider the work entailed over just changing the skintone codes.

It'll be easier if you note the codes for each skin and assign them to certain sims, like this:
7304bb24-470b-1612-5baf-39b948590bee:
Lola Curious
Chloe Curious
Johnny Smith

That way, you'll just be editing one skin group at a time without redundancy.

If you delete custom and geneticized skins without replacing the codes for your existing sims, this will result in neighborhood corruption. This is because the sim's DNA will refer to a skintone that's no longer there. In-game, you'll find that the sims will revert to the default skintones, but the reality is that they're not actually there. When they have children, those corrupt genetics will pass on to them and so on and so forth (scroll down to neighborhood corruption).

I have so many sims using the custom skins, and also townies, so I'd somehow rather avoid this
Although if I do go down this method I have some questions
Do you have to change each skintone individually for each sim or can you do it in groups?
Also do I need to change the DNA for the sims recessive skintone or skintone range, or do these properties not do anything?

Regarding replacing skins, is there possibly a way to do it simply? I was thinking what if I take Lilith's skin and just sort of overwrite the old skin by changing the name, and going into SimPE and changing the family (dstring) value to the old skin's value? Would that be at all feasible?

Ok so I tried out the method of replacing the family dtstring for one skin and it totally worked. I went back into sims and the character that I knew had that skin was wearing the new one.

I don't know if there are any consequences or side effects though, all the property set values list the skintone dtstring as the old one because I couldn't be bothered changing them all, and I don't know if that will change or affect anything

I don't want to corrupt my game above all

It won't cause neighborhood corruption just corruption to the DNA that your sims will pass on. I would check under the More Tab of each sim and check that it corresponds to what the skin should be. You might want to check receives as well.

What do you mean by the More tab? The DNA? What am I looking for and what is this in relation to?
Also what is receives?

Sim Browser- Open a sim you know you changed the skin on in there is MORE under MORE is DNA. In two spots actually as under there is also character file and that also has some DNA. But first I would just check under the first DNA tab, that shows the string of code of the skin that your sim is wearing. There are dominant and recessive. (sorry typo on recessive)

I'm not changing the sims DNA, so that part will stay the same and isn't the problem I don't think. What I'm changing is where the code leads to, I'm changing the actual skin itself.
Basically I open the skin I want in SimPE and the skin I want to replace, and I replace the family (dtstring) from the new one with the old one, and delete the old file all together.