Originally Posted by
Xeno
Don't think the OP means traditional SDC but rather using the 24 hour rule related to the original purchase.
Ah, I got tripped up by OP's use of the phrase "same day change" in the thread post/title but on re-reading the post body I think you're right.
Originally Posted by
third_wave
Google flights usually includes Basic Economy (E) fares. If you click through from Google Flights you should be able to see what Google saw. It's just a scraper from the website. There is nobody to call at Google Flights.
While this is true, OP is asking about upfaring to first/biz so unlikely to be looking at E fares, which would still show up at DL's site anyway so still doesn't explain the discrepancy.
I still think caching is a likely explanation, but would still be interested in seeing specific route/dates and/or screenshots...