This is what Pam told me, and I followed it and it worked. It's written for Gunther but can be used on any sim.
1. Go to Edit Town and take control of the Goth household.
2. Send Gunther to a mirror or dresser to change appearance or plan outfit.
3. While in that version of CAS, click on the button with 3 dots and choose save to bin (or library).
4. Exit CAS.
5. Go back to Edit Town and go into CAS.
6. Find Gunther Goth under Pre-made Sims (button in lower left corner).
7. Change Gunther's skin to blue. Leave his traits and everything else the same.
8. Add the new Gunther to the Goth household through Edit Town.
9. Take control of Goth household and delete old Gunther.
They are by age and gender in CAS pre-made sims too.