This update from the Thrifty Traveler about a recent wave of $500 upgrade offers:
Prices for upgrades tend to bottom out within three weeks of departure – and especially within the final week.
If you don't see a price you like, look hard at the seat map in Delta One. If it's fairly full, don't count on a bargain. But if there are plenty of open seats, it's easily worth checking back again.
There's no question this trend is most pronounced on flights to and from Europe, but we've seen great upgrade deals between the U.S. and Asian cities like Tokyo-Haneda (HND) and Seoul (ICN), too.
These excellent upgrade offers are getting extended whether you booked a standard main cabin economy seat, Delta Comfort Plus, or Delta Premium Select – though higher-paying customers may see the lowest upgrade rates
There's one exception: Delta basic economy fares are ineligible for these cabin upgrades, period.