Issue with loading 4.6 project into 4.71 - Lighting all messed up

Hi All.

I’ve just been working on an interior in 4.6, slowly building the meshes and lighting up so it looks really nice, then being excited about 4.7.1 jumped right into that only to open it up just now and find all the shadows and general lighting seems messed up. I tried rebuilding the lighting but even though it seems to take longer the sun shadows are just a blurred out mess instead of sharp and it seems like theres artifacts in all the corners everywhere.

Had to go back and carry on in 4.6 but hoping someone knows about this and can give me a solution. I checked all the lightmass settings and mesh lightmap resolutions - everything looks the same as the copied over 4.6 scene so not sure why it would all of a sudden build badly in this way?

Thanks

Karl D

Anyone have any ideas? Been messing around with this for hours now and it just seems like 4.7.1 is messed up to me!