What I recieved from Delta. Read the last sentence.
As a Diamond Medallion member, you receive unlimited complimentary
upgrades which are subject to availability. They may not be available
on all flights or in all markets and are not valid for travel to/from,
or to Transatlantic or Transpacific destinations. Complimentary
upgrades are not available on flights to/from South American cities that
are serviced with our BusinessElite product (i.e., Buenos Aires,
Argentina; Rio de Janeiro and Sao Paulo, Brazil; Santiago, Chile; Lima,
Peru; Caracas, Venezuela). For travel booked using a SkyMiles Companion
Certificate, neither the paid primary passenger nor the companion
passenger will be eligible for complimentary upgrades.
When making reservations on delta.com, over the phone, at the airport,
or through a travel agent, members will receive an automatic
complimentary upgrade request. Once the reservation is ticketed, the
upgrade will be cleared based on the confirmation window, availability,
fare class, and time of request.
If available, upgrades for all Medallions (Diamond, Platinum, Gold, and
Silver) are cleared at time of ticketing for Y class fares. For Diamond
Medallions, they clear 5 calendar days in advance for B, M, S, H, Q, K,
L, U, T, X, V and E class fares. Clearance starts with B class and
continues in descending order. If Medallion level and fare class are
equal, time of ticketing will be the tie breaker.