I had the exact same problem. What I did was copy the entire family with house to the library, not move them out. Once I had a copy in my library, I deleted the original family and house to just an empty lot. Then, I took the freshly copied family (with house) and placed them on the freshly demolished lot. Family tree was still intact, so were careers, and so was individual sim inventory AND family inventory. The only things that weren't safely copied were relationships with people outside the house and paintings in the house that were painted by sims.