If you don't clear 24 hours prior to departure, then unfortunately the mileage upgrade doesn't transfer to the airport standby list. The only thing that does are Global Upgrade Certificates. It's unfortunate, because the common knowledge is that it's because of the time it takes to pull the miles from the account while trying to board an international flight... but Ex-AMS flights, managed by KLM, on DL metal are a relatively quick transaction at the transfer desk, just not a resource available within DL's home network, apparently.

I had a very sympathetic DL agent on the phone once when I was trying to figure out how to get on a SEA-AMS flight that was wide open in Delta One, and yet at hour 25, nothing had opened for an upgrade with miles and the clock was running out-- they had even volunteered to call Revenue Management to double check (and admitted they were hesitant to because they were overzealous for another passenger with that desk recently and didn't want to overplay the courtesy check) but they weren't available in a reasonable amount of time.

The DL DM agent though did say if she were me, she'd get to the airport 3 hours prior to departure and hover around the gate until the agents look like they're not busy enough to take walk ups-- I also bought some Starbucks gift cards for anyone who was possibly going to do me a favor, and offered them up front as a thank you for their time, not whether or not we were successful.... but I asked the agent if she had the time to make the phone call to clear a mileage upgrade at the gate... very patiently, and politely... and she told me to wait near the seats on the side of the podium. While I waited, she confirmed they would have seats open if they were processing the airport standby list with me on it, and then explained the request to a Red Coat. I informed her, that some kind advice from a DL agent was to politely ask, and see if they were willing to make the phone call-- if not, no big deal, if so, thank you very much.

She seemed surprised, nobody had ever asked for a mileage upgrade gate clearance before, and she was sort of excited to see if we could make it happen before the doors needed to close. She called, explained, and then was put on hold for a pretty long time, I'd say about 10 minutes, felt like 15 but maybe because I was anxious, but somewhere in there she just turned around with the phone on her ear and gave me the thumbs up. I was relieved, relaxed and excited! She still stayed on the phone for another 5+ minutes or so, which I assume was the ticketing, and she needed that, and for me to wait, so she could print a boarding pass. That was a fun experiment! But I also have boarded plenty of flights lately where there would have been no resources and absolutely no time to make that kind of magic happen. YMMV, but that's one anecdote!