Cover implementation practices and suggestions

[j9t]

Dto.: for example, per Dylan Paul (thank you!):

I am seeking to actually implement some of the concepts and systems you talk about. I am not sure if you have talked about this before but I think the act of implementing some of these concepts to a pre-existing unoptimized project is something I would like some more insight on.

Should I make all previously unoptimized CSS conform to an architecture system like BEM (if I were to choose to implement BEM), or would it make more sense to only make future CSS have these optimizations? Is it a headache trying to iteratively introduce newly embraced formatting, coding, or naming standards? Perhaps the best way is to take a week and sit down and back-fill all of the old CSS and make it optimized?