Just to be clear, this only applies to Delta-operated segments of your trip. Complimentary upgrades for Delta Medallion members on Aeromexico-operated flights are day of departure only, regardless of who marketed the flight.

So, imagine you're flying CVG-DTW-MEX where CVG-DTW is operated by Delta but DTW-MEX is operated by AM. If you bought a AM-marketed ticket, you'd have to wait until DoD to upgrade to C+ or F on CVG-DTW even if your Medallion benefits entitle you to upgrades upon booking or at the window. You'd be subject to DoD upgrades for DTW-MEX even if you had booked a DL marketed flight, since in this example, it's AM operated.