I guess it really depends. Borking people’s saves would be a bad route to go down, unless they were making some fairly major additions to the worlds. I’d be fine keeping the existing neighborhoods in tact and adding new, larger neighborhoods to the worlds. Not sure if that would affect saves like modifying existing neighborhoods would - or even if that is possible within this game. We were told originally that they couldn’t tack on new locations in the worlds, which is why we got MP as a 4 lot world dressed to resemble a willow creek neighborhood.
I’m not one who really prefers they focus on *new* worlds. They focus on the wrong areas with them IMO, and they usually wind up being heavily decorated pockets with few lots and little gameplay. I’ve found SM to be the most unappealing world to play in, as there is more to look at than actually do. Brindleton Bay and Windenburg aren’t far behind, but they at least offered some breadth with those maps. However, even those larger maps have tons of dead space that could have adequately fit several lots. Instead it’s reserved for static decoration that your sim will never even interact with or be aware of (I am not suggesting they make sims aware of the world deco). They need to focus on making a world, not a collection of small subdivisions.