@Bri
I have followed the instructions you posted, and they did work.
However, since the pool patch (I think), I have not been getting the option to move the split off household into a new house. The menu just doesn't come up. I can move a sim out, but that sim just disappears.
My work-around has been to save my entire family to the gallery, go back to my game, then go to manage household and click on a house, select the saved family to move into the new house, and then delete everyone except the sim I want to move in. I move in the sim and then go back to the original house and move out the sim -- apparently into the ether.
Unfortunately, although my sim has retained her skills and career, she has no relationship with the rest of her family.
The only other thing I can think of is to move her into a household in town, and then delete all the other members. She would keep her relationships then, I think.