It should be the same upgrade class as other D1, which is OY.
Looking at the T&Cs, it states:
"Certificate may be used to upgrade...operated flights on published fares booked in P, A, G, W, Y, B, M, H, Q, K, L, U, T, X or V class and upgrade into RY/OY class."
Applicable upgrade booking classes for cabins are: OY, RY, SY
Also per the posted waitlist info on the DL website:
https://www.delta.com/content/www/en...ml#waitlisting
There is nothing that excludes D1 on the A350. They are still D1 and the difference between D1 seats is not broken out.