I've had the game do that to me before, specifically when I tried to add the Pancakes' toddler to the Pancakes I already had in the game. The game recognized that the toddler already had parents, so I couldn't just change them, I needed to make the kid unrelated to its current parents, then add the new relationships. It was a pain in the butt and I ended up just switching out the whole family, which you said you don't want to do.
There may not be an easy way to do it, but if you can, find the Sim's parents and make them unrelated first.
