I don't know if this is still working or if it was patched out, but some month ago I tested loading with editor and changing the level used in GM Mode and when restarting / reloading GM Mode the changes made with the editor ( like terrain height , texture, placed placeables ) were applied with all things placed via GM Mode like NPCs, surfaces still intact. BEWARE if you try out this, it could screw up your level or cause technical issues. But if you are curious, backup your stuff and try...