Hey guys, someone has made a plugin to do this:
GitHub - Cesio137/UE4-GraphicsRHIManager
You can read about it in this thread:
How to toggle between DX10, DX11, DX12 and Vulkan via Blueprint? - Unreal Engine / Rendering - Unreal Engine Forums
It requires the game to be restarted after the change (im sure there has to be a way around this, but take it or leave it 
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