Parmandur
Book-Friend, he/him
I mean, exactly. The extremely rough guesstimate guidelines are not any more guesstimate than they were before.HA HA!
Of course the CR system is backwards-compatible.
It won't work in OneD&D, just like it never worked in 5e.
Problem solved.
What will be interesting is if WotC comes up with a better encounter building math for OneD&D. That would be bigger than any PC build changes.