Here's a Reader's Digest version of the changes. (less than one full page with changes highlighted)
https://smf.org/standards/sa/2020/SA2020_cover.pdf
If I were in the market for a new helmet I would buy a 2020, not because it's vastly safer (in fact I have very little confidence that it would even be a different helmet from most manufacturers), but because I could get 10-11 years of use out of it with proper care instead of only 5-6 if I bought a 2015.