Originally Posted by
Gig103
The reason it's challenging in economy is that there needs to be the same fare on your new flight. So buying far in advance on an a 'deep discount economy' fare is going to be hard to find day-of travel. However, when flying in first class, any (non-Delta-One) space is allowed for SDC. There's a helpful wiki page about it on Flyertalk:
Same-day confirmed and same-day standby options
note that paid (not complimentary upgraded) Comfort+ is also eligible for SDC if there's an available seat, without regard for the underlying coach inventory