How to modify BRDF.ush file to switch to phong specular model? (instead of Unreal's GGX default)

Do you know of a way to use Unreal’s phong or Blinn specular model instead of the default/GGX?? I read it involves modifying the BRDF.ush and ShadingModels.ush files?
I read that this is possible…-cgi-film-look
How would I go about doing this and where should I start? (like which line specifically do I modify, tutorials,documentation, etc.)

Thanks in advance.

I asked the person who wrote this blog the same question…ading-model-1/

The blog was on how to create a custom toon shading model and it looked promising enough, but unfortunately they couldn’t help with changing the specular model. This seems to be not well documented and any info I found on BRDF is from 2014, so very outdated. (I’m using Unreal 4.20)