Two of my families would like to move into this duplex (lot A. no CC version).

However, they wanted me to make some changes to the bottom floors. I switched lot zoning to residential, and then:
- Removed a few walls.
- Added a few half-walls
- Replaced a few doors with arches.
- I only changed the bottom floor.


Now the game won't let me switch back to apartmentbase.

Things I checked:

- Apartment mat is facing the right way.
- I didn't knock any holes into the walls separating the apartments.
- The half-walls and arches are both Maxis. I didn't use any floor dividers.
- Tried removing the arches and putting in doors
- Tried removing half-walls
- Tried replacing the backyard fence
- Tried deleting the stairs and filling in the floor tiles.

Can someone find the issue?

You can't use arches in apartments, they count as walls, just like the floor dividers do.

I've had arches in previous apartments though. The wikia also confirms:

https://sims.fandom.com/wiki/Game_g...oors_and_arches

It was the doors leading out to the backyard. This also happened if I switched the lotzoning of the original unmodified lot.

I'm curious how the original creator got around this.

You solve the problem with the backyard being on a different level by putting the floor dividers around the outside of the door leading outside on the same level, before the stairs. Then that counts as a wall and the backyard is classified as common area, but isn't accessible without going through the apartment. But it looked like you had done that already.

Yes, that's what I ended up doing. I wonder if the CC version had an invisible custom floor divider and the no-CC version just forgot about it.

I think the usual way is you move_objects the stairs onto the tile where the floor divider was, and the floor divider becomes invisible. Dunno why it broke, though.