Cannot edit pre-existing sims

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.