DOT won't care so long as UA fulfilled the ticket requirement of getting OP from MSP-SFO. Even class of service as booked won't matter so long as UA redeposit the cert (which is the fare difference as it were, between F and C).

I suspect that UA didn't bump OP for a GS UG, but rather for a paid F passenger who changed to the ORD 3-class either through direct change (F is changeable) or who misconnected.

Annoying, but don't see AA or DL doing anything different. Or, DOT giving a hoot.