I just checked the official rules (https://www.delta.com/content/www/en_US/skymiles/about-skymiles/medallion-program/medallion-benefits/unlimited-complimentary-medallion-upgrades.smt-diamond.html) and it states

Medallion members and their companions must be booked in a revenue class of service: Y, B, M, H, Q, K, L, U, T, or E.

Based on this all paid fare classes are still valid for companion upgrades.