Cannot edit pre-existing sims

Hi! I'm not sure about what "relationship" in CAS (Create-a-Sim) you mean. Do you mean a friendship/romance level or a genetic/household member relationship? To change a genetic or household relationship, try the regular cheat. Open cheat window (CTRL+Shift+C, press/hold with one fluid motion in that order) in the play mode and type: testingcheats on (testingcheats true should work too, I've just always used testingcheats on). Then type: cas.fulleditmode. (I put the cheat in before I enter CAS.) This allows you to fully edit a sim in CAS. You should be able to change their household status (and bodies, likes, etc.). If you are looking to make married sims unmarried without a divorce, that's a different story. Shift click on the sim(s) in the play mode (not in CAS), and then more choices, marriage and then clear marriages. Do it for their spouse, too. Enter CAS to make sure that the change took place there too, if not, correct it. Don't forget to "save" after your changes. Tip: I enter the cas.fulleditmode cheat in the play mode at the start of every time I play. This allows me to go into CAS during my entire game play, session, even after changing households/lots to quickly make changes to a sim. 

viviplaysims​  A couple of things.  You don't need testingcheats for the cas.fulleditmode cheat.  Changing relationships in CAS is hit and miss for some reason.  Sometimes you can, sometimes you can't.  One thing you can't do is set a parent/child relationship if both sims are the same age.  There is a workaround for that case (age one sim up or down, set the relationship, age the sim back to what they were.) 

Imo, using MCCC is the easiest way to do it.  Just use the options in the Relationships module.   So, if you don't play with mods, just add the mod, start the game and use the mod to reset the relationship, save and shut down, remove the mod and restart.

Hope this helps.

Hi viviplaysims​ 

to my knowledge, you'll need a mod for this cheat. either allcheats or mccc