[General] Changing terrain height with doodads adapting to new height

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Hi,

I have an area with thousands of doodads which for some design reasons were made with a terrain cliff height difference. However, now I kind of want to get rid of that and use Same level for the whole area. The problem is, when I try to do that the doodads act in various odd ways. Some of them stay at the old height level, others move to the new and anything in between.

Is there a way to smoothly change cliff level for an area and make all doodads adapt to the new height (decrease) automatically?