Some people were having trouble with this, so here it is.
With MCCC installed, click on the desired Sim you want to replace
Choose MC Command Center
> MC CAS
> Set Appearance by Template
> Copy/Paste
> Paste Tray Sim
> Any Saved Sims
At this point all the households in your gallery will appear on a list.
Choose the desired Household then the name of the Sim.
Then Select what you want to replace;
Appearance, Clothing, Gender, Name, Personality
Click on Done and it should work. π
(There can be glitches on some occult sims, so save before doing this!)