I was wondering how yo can change the falloff or distance to the main character, to tell the Foliage when it should change the Level of Detail?
I made this short video to explain the problem:
You can just Edit the **Foliage Mesh **(right click on it inside the viewport >> Edit…) and, inside the Detail Panel, uncheck Auto Compute LOD Distance.
Then, you can select each LOD (in LOD Picker) and set a new value to the Screen Size.
More details in Unreal Engine Documentation.
Hope it helps!