My wife and I go between ATL and SFO almost every month, and the plane has always been what they "advertised" it would be. The equipment has changed over the years along with schedule changes, but not last minute/at the gate stuff like the OP is talking about. Currently we take a 757 out there and a 764 on the return, just as they're listed in DLs flight schedules. In fact, we just did that trip last week.

Currently, DL has 7 flights a day between ATL and SFO--three are on 764s, three on 757s and one on a 763.