Although I would expect the status on any DL reservation to reflect what it is on the day of travel, it is possible that your status on a partner airline's reservation could continue to be what it was at time of purchase. I doubt they sync them in real time. But even if the reservation had the correct status, the overseas lounges often want to see a physical card, and your plat card would show expired.

Unfortunately I have no direct experience with such a status change between purchase and flight, except on CO. In that case, if you got an upgrade prior to your status going away, the upgrade stuck. But day of flight perks reflected your status on that day.