chiggeuk If you move your old saves back and get a notification that a file already exists with X name, you skip that file and rename one of the two that conflict. It technically doesn't matter which one, but I would rename the one you're moving back (not the one that's already in the saves folder) in case you accidentally get the name wrong. It's more important that the new save have the correct name.
Please post a screenshot of the error you see when you try to load a household. And let me know whether this is with a new save or an older one. I'm not sure whether you're saying the new save produces this error, or you're now able to try to load an old save but see the error when you choose a household.