Set an expertflyer alert for the required KE inventory if you have a subscription.

The 24 hours means that DL just needs to process the ticket/upgrade by 24 hours from departure... not that KE will release more inventory in that period. It is possible that KE could release more upgrade/award inventory in the days/weeks leading up to the flight, but as others mentioned they are notoriously stingy with only giving 1 or 2 award spots per flight.

However, you mentioned you are booking a B fare... which means you only need the KE upgrade inventory Z, not award inventory O so that should be much more readily available. For example, I see lots of flights where it is O=0 award but Z=9 upgrade. If you are buying a B fare, make sure the agent is looking for the proper Z inventory on the KE flight number. They probably don't do this very often or at all so might be looking for award O instead or looking at the DL flight for inventory instead of the KE number.