I don't know if it was SOP, but about a year ago I was on one of the regular MSP-SEA morning flights. They replaced the regular 757 with a 747. Ended getting a seat on the upper deck!!

At least some of the FAs didn't know until they got to the gate. They added quite a few extra FAs to the flight.

I talked to the pilot for a bit and he said they would pick up the plane that just flew NRT-SEA and fly it back to MSP(w/pax) for regular maintance.

Quite a pleasent unexpected experience!!