Just use caution when increasing household size. If you go into CAS with that household you WILL lose sims over the 8 limit. There isn't a mod so far to prevent that (at least not that I've seen anywhere). If you avoid entering CAS until you can split them back up, you should be safe.
Here's what I do when moving related sims to a new save, I use the MCCC mod, once the families are moved I use the mod to add relationships. This way I can preserve family relationships without having to have everyone all in the same household. You can set parental/child and sibling relationships with the mod. It can also be used to set active friendship/romance levels.