I doubt you can do that.
You could try and compare source between versions, but the changes removing light propagation volumes and getting the emissive channel to bake have nothing to do with sm5.
There’s a lot involved in the rendering pipeline and as far as I have seen the source is less than poorly commented.
Updates and changes are hard to track (other than using /diff on 2 file sets).
Still there’s no point. Use an engine version that works (.25 I guess).