If you use the scripted method, check that the features and groups used are the same in both tilesets (IIRC there are a few unique ones).
Static objects need to be duplicated in the toolset, obviously.
You also need to "move" any persistent objects which can be altered by gameplay.
For creatures, ActionJumpToLocation(new area, GetPosition, GetFacing) works fine. Alternatively, duplicate the creatures in the toolset, but I find that's more work.
Placeables and items can't move. The simplest approach is copy-and-destroy everything, remembering to copy inventory and variables in the process.