I had the "jump to 10m" problem last night in Unlimited (even lowering raised it by 10m).  Unfortunately it seems to have set the terrain height on which to base the +/- 15m at the unwanted raised level so if I now try to lower the top of the hill I actually only go down 5m below the original terrain.

If this is so, the only way I will be able to hollow out a new cave will be to raise 15m around the edge from the false level thus making the hill 25m above its original level which will look awful.