The exact thing happened to me about 3 years ago with a different (better) outcome. After a mechanical at JFK resulted in an extra night in New York, when I was rebooked for the next day I was in Delta J from JFK/NRT but moved to SQ and booked in F. I was stunned - especially since I was on an M class fare upgraded with miles.

Of course each time I sipped a bit of Dom champagne the attentive flight attendants refilled it so I don't remember that much about the flight - other than the pajamas were soft and the caviar was tasty.