I got the same thing on Monday from OMA to ORD.

I'm Gold and my upgrade cleared at 24 hours. No coupon needed as it's under 500 miles.

I like to get a paper BP at the kiosk, but it spit out a "go to the desk" chit instead. Desk agent printed a BP in exit row Y. He explained that due to light load there had been an equipment swap to a smaller plane with no F.

Later at the gate, the agent paged me and handed me a $300 voucher.