Thanks for your responses, guys! :blush:
@Emmely_Bettine If you save Sims to your library and place them in a new save, they retain all of their skills, their job, their relationships with fellow household members, their aspiration reward traits and points, and I'm pretty sure also their current age. They will lose all other relationships with Sims not in the household, even if you place both families in the same save - they will also lose familial relationships with anyone not in the household you saved them in. They will lose relationships with townies because the new save will have new townies. You'll have to repopulate the new save, and place new lots. The new save will be brand new, with every lot and all premades in their as they were when you started your first save.