@ConverseFox A big ‘duh’ moment - now it works 
Edit: Had a moment to test a bit. Works quite nicely. Even creasing. Many bugsplats though…
I get the impression its meant to be an editor function (first). Adding subd levels to an object changes the object in the asset browser as well. Updating from a BP crashes the game. Hoping it can be used ingame as well - sort of a lod function - maybe great for archviz. Very curious to see whats coming form Epic…
Edit2: its in the fbx import options so seems not for runtime changes (at first): https://docs.unrealengine.com/en-us/Engine/Content/FBX/ImportOptions