Check to make sure but either of these actions should add them to your 'played households' and protect them from deletion/replacement by the game.
If you want make sure any particular sim household that you like but don't play doesn't get deleted, open it and save or just 'favorite' it in the households. (If you want to you can save the lot to your library before building over it, in case you may want to re-add it somewhere later). Then you can use or build on or move into those lots. they just won't live in one of the houses. They will then 'live' in the played/unplayed households, and they will still be 'townies' and go to businesses, public lots, etc. A few things to point out, in case you didn't realize this: As certain expansions do come with additional worlds, which will also be a combination of retail/public plus already built (filled and unfilled) residential homes, plus a sprinkling of empty lots.
